Patchwork [PATCHv7,3/3] vhost_net: a kernel-level virtio server

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Submitter Michael S. Tsirkin
Date Nov. 3, 2009, 5:24 p.m.
Message ID <20091103172422.GD5591@redhat.com>
Download mbox | patch
Permalink /patch/37521/
State RFC
Delegated to: David Miller
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Comments

Michael S. Tsirkin - Nov. 3, 2009, 5:24 p.m.
What it is: vhost net is a character device that can be used to reduce
the number of system calls involved in virtio networking.
Existing virtio net code is used in the guest without modification.

There's similarity with vringfd, with some differences and reduced scope
- uses eventfd for signalling
- structures can be moved around in memory at any time (good for
  migration, bug work-arounds in userspace)
- write logging is supported (good for migration)
- support memory table and not just an offset (needed for kvm)

common virtio related code has been put in a separate file vhost.c and
can be made into a separate module if/when more backends appear.  I used
Rusty's lguest.c as the source for developing this part : this supplied
me with witty comments I wouldn't be able to write myself.

What it is not: vhost net is not a bus, and not a generic new system
call. No assumptions are made on how guest performs hypercalls.
Userspace hypervisors are supported as well as kvm.

How it works: Basically, we connect virtio frontend (configured by
userspace) to a backend. The backend could be a network device, or a tap
device.  Backend is also configured by userspace, including vlan/mac
etc.

Status: This works for me, and I haven't see any crashes.
Compared to userspace, people reported improved latency (as I save up to
4 system calls per packet), as well as better bandwidth and CPU
utilization.

Features that I plan to look at in the future:
- mergeable buffers
- zero copy
- scalability tuning: figure out the best threading model to use

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 MAINTAINERS                |    9 +
 arch/x86/kvm/Kconfig       |    1 +
 drivers/Makefile           |    1 +
 drivers/vhost/Kconfig      |   11 +
 drivers/vhost/Makefile     |    2 +
 drivers/vhost/net.c        |  633 +++++++++++++++++++++++++++++
 drivers/vhost/vhost.c      |  970 ++++++++++++++++++++++++++++++++++++++++++++
 drivers/vhost/vhost.h      |  158 +++++++
 include/linux/Kbuild       |    1 +
 include/linux/miscdevice.h |    1 +
 include/linux/vhost.h      |  126 ++++++
 11 files changed, 1913 insertions(+), 0 deletions(-)
 create mode 100644 drivers/vhost/Kconfig
 create mode 100644 drivers/vhost/Makefile
 create mode 100644 drivers/vhost/net.c
 create mode 100644 drivers/vhost/vhost.c
 create mode 100644 drivers/vhost/vhost.h
 create mode 100644 include/linux/vhost.h
Eric Dumazet - Nov. 3, 2009, 6:03 p.m.
Michael S. Tsirkin a écrit :
> +static void handle_tx(struct vhost_net *net)
> +{
> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> +	unsigned head, out, in, s;
> +	struct msghdr msg = {
> +		.msg_name = NULL,
> +		.msg_namelen = 0,
> +		.msg_control = NULL,
> +		.msg_controllen = 0,
> +		.msg_iov = vq->iov,
> +		.msg_flags = MSG_DONTWAIT,
> +	};
> +	size_t len, total_len = 0;
> +	int err, wmem;
> +	size_t hdr_size;
> +	struct socket *sock = rcu_dereference(vq->private_data);
> +	if (!sock)
> +		return;
> +
> +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> +	if (wmem >= sock->sk->sk_sndbuf)
> +		return;
> +
> +	use_mm(net->dev.mm);
> +	mutex_lock(&vq->mutex);
> +	vhost_no_notify(vq);
> +

using rcu_dereference() and mutex_lock() at the same time seems wrong, I suspect
that your use of RCU is not correct.

1) rcu_dereference() should be done inside a read_rcu_lock() section, and
   we are not allowed to sleep in such a section.
   (Quoting Documentation/RCU/whatisRCU.txt :
     It is illegal to block while in an RCU read-side critical section, )

2) mutex_lock() can sleep (ie block)

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Gregory Haskins - Nov. 3, 2009, 6:08 p.m.
Eric Dumazet wrote:
> Michael S. Tsirkin a écrit :
>> +static void handle_tx(struct vhost_net *net)
>> +{
>> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
>> +	unsigned head, out, in, s;
>> +	struct msghdr msg = {
>> +		.msg_name = NULL,
>> +		.msg_namelen = 0,
>> +		.msg_control = NULL,
>> +		.msg_controllen = 0,
>> +		.msg_iov = vq->iov,
>> +		.msg_flags = MSG_DONTWAIT,
>> +	};
>> +	size_t len, total_len = 0;
>> +	int err, wmem;
>> +	size_t hdr_size;
>> +	struct socket *sock = rcu_dereference(vq->private_data);
>> +	if (!sock)
>> +		return;
>> +
>> +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
>> +	if (wmem >= sock->sk->sk_sndbuf)
>> +		return;
>> +
>> +	use_mm(net->dev.mm);
>> +	mutex_lock(&vq->mutex);
>> +	vhost_no_notify(vq);
>> +
> 
> using rcu_dereference() and mutex_lock() at the same time seems wrong, I suspect
> that your use of RCU is not correct.
> 
> 1) rcu_dereference() should be done inside a read_rcu_lock() section, and
>    we are not allowed to sleep in such a section.
>    (Quoting Documentation/RCU/whatisRCU.txt :
>      It is illegal to block while in an RCU read-side critical section, )
> 
> 2) mutex_lock() can sleep (ie block)
> 


Michael,
  I warned you that this needed better documentation ;)

Eric,
  I think I flagged this once before, but Michael convinced me that it
was indeed "ok", if but perhaps a bit unconventional.  I will try to
find the thread.

Kind Regards,
-Greg
Gregory Haskins - Nov. 3, 2009, 6:14 p.m.
Gregory Haskins wrote:
> Eric Dumazet wrote:
>> Michael S. Tsirkin a écrit :
>>> +static void handle_tx(struct vhost_net *net)
>>> +{
>>> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
>>> +	unsigned head, out, in, s;
>>> +	struct msghdr msg = {
>>> +		.msg_name = NULL,
>>> +		.msg_namelen = 0,
>>> +		.msg_control = NULL,
>>> +		.msg_controllen = 0,
>>> +		.msg_iov = vq->iov,
>>> +		.msg_flags = MSG_DONTWAIT,
>>> +	};
>>> +	size_t len, total_len = 0;
>>> +	int err, wmem;
>>> +	size_t hdr_size;
>>> +	struct socket *sock = rcu_dereference(vq->private_data);
>>> +	if (!sock)
>>> +		return;
>>> +
>>> +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
>>> +	if (wmem >= sock->sk->sk_sndbuf)
>>> +		return;
>>> +
>>> +	use_mm(net->dev.mm);
>>> +	mutex_lock(&vq->mutex);
>>> +	vhost_no_notify(vq);
>>> +
>> using rcu_dereference() and mutex_lock() at the same time seems wrong, I suspect
>> that your use of RCU is not correct.
>>
>> 1) rcu_dereference() should be done inside a read_rcu_lock() section, and
>>    we are not allowed to sleep in such a section.
>>    (Quoting Documentation/RCU/whatisRCU.txt :
>>      It is illegal to block while in an RCU read-side critical section, )
>>
>> 2) mutex_lock() can sleep (ie block)
>>
> 
> 
> Michael,
>   I warned you that this needed better documentation ;)
> 
> Eric,
>   I think I flagged this once before, but Michael convinced me that it
> was indeed "ok", if but perhaps a bit unconventional.  I will try to
> find the thread.
> 
> Kind Regards,
> -Greg
> 

Here it is:

http://lkml.org/lkml/2009/8/12/173

Kind Regards,
-Greg
Eric Dumazet - Nov. 3, 2009, 6:51 p.m.
Gregory Haskins a écrit :
> Gregory Haskins wrote:
>> Eric Dumazet wrote:
>>> Michael S. Tsirkin a écrit :
>>>> +static void handle_tx(struct vhost_net *net)
>>>> +{
>>>> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
>>>> +	unsigned head, out, in, s;
>>>> +	struct msghdr msg = {
>>>> +		.msg_name = NULL,
>>>> +		.msg_namelen = 0,
>>>> +		.msg_control = NULL,
>>>> +		.msg_controllen = 0,
>>>> +		.msg_iov = vq->iov,
>>>> +		.msg_flags = MSG_DONTWAIT,
>>>> +	};
>>>> +	size_t len, total_len = 0;
>>>> +	int err, wmem;
>>>> +	size_t hdr_size;
>>>> +	struct socket *sock = rcu_dereference(vq->private_data);
>>>> +	if (!sock)
>>>> +		return;
>>>> +
>>>> +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
>>>> +	if (wmem >= sock->sk->sk_sndbuf)
>>>> +		return;
>>>> +
>>>> +	use_mm(net->dev.mm);
>>>> +	mutex_lock(&vq->mutex);
>>>> +	vhost_no_notify(vq);
>>>> +
>>> using rcu_dereference() and mutex_lock() at the same time seems wrong, I suspect
>>> that your use of RCU is not correct.
>>>
>>> 1) rcu_dereference() should be done inside a read_rcu_lock() section, and
>>>    we are not allowed to sleep in such a section.
>>>    (Quoting Documentation/RCU/whatisRCU.txt :
>>>      It is illegal to block while in an RCU read-side critical section, )
>>>
>>> 2) mutex_lock() can sleep (ie block)
>>>
>>
>> Michael,
>>   I warned you that this needed better documentation ;)
>>
>> Eric,
>>   I think I flagged this once before, but Michael convinced me that it
>> was indeed "ok", if but perhaps a bit unconventional.  I will try to
>> find the thread.
>>
>> Kind Regards,
>> -Greg
>>
> 
> Here it is:
> 
> http://lkml.org/lkml/2009/8/12/173
> 

Yes, this doesnt convince me at all, and could be a precedent for a wrong RCU use.
People wanting to use RCU do a grep on kernel sources to find how to correctly
use RCU.

Michael, please use existing locking/barrier mechanisms, and not pretend to use RCU.

Some automatic tools might barf later.

For example, we could add a debugging facility to check that rcu_dereference() is used
in an appropriate context, ie conflict with existing mutex_lock() debugging facility.


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Gregory Haskins - Nov. 3, 2009, 7:50 p.m.
Eric Dumazet wrote:
> Gregory Haskins a écrit :
>> Gregory Haskins wrote:
>>> Eric Dumazet wrote:
>>>> Michael S. Tsirkin a écrit :

>>>> using rcu_dereference() and mutex_lock() at the same time seems wrong, I suspect
>>>> that your use of RCU is not correct.
>>>>
>>>> 1) rcu_dereference() should be done inside a read_rcu_lock() section, and
>>>>    we are not allowed to sleep in such a section.
>>>>    (Quoting Documentation/RCU/whatisRCU.txt :
>>>>      It is illegal to block while in an RCU read-side critical section, )
>>>>
>>>> 2) mutex_lock() can sleep (ie block)
>>>>
>>> Michael,
>>>   I warned you that this needed better documentation ;)
>>>
>>> Eric,
>>>   I think I flagged this once before, but Michael convinced me that it
>>> was indeed "ok", if but perhaps a bit unconventional.  I will try to
>>> find the thread.
>>>
>>> Kind Regards,
>>> -Greg
>>>
>> Here it is:
>>
>> http://lkml.org/lkml/2009/8/12/173
>>
> 
> Yes, this doesnt convince me at all, and could be a precedent for a wrong RCU use.
> People wanting to use RCU do a grep on kernel sources to find how to correctly
> use RCU.
> 
> Michael, please use existing locking/barrier mechanisms, and not pretend to use RCU.

Yes, I would tend to agree with you.  In fact, I think I suggested that
a normal barrier should be used instead of abusing rcu_dereference().

But as far as his code is concerned, I think it technically works
properly, and that was my main point.  Also note that the usage
rcu_dereference+mutex_lock() are not necessarily broken, per se:  it
could be an srcu-based critical section created by the caller, for
instance.  It would be perfectly legal to sleep on the mutex if that
were the case.

To me, the bigger issue is that the rcu_dereference() without any
apparent hint of a corresponding RSCS is simply confusing as a reviewer.
 smp_rmb() (or whatever is proper in this case) is probably more
appropriate.

Kind Regards,
-Greg
Michael S. Tsirkin - Nov. 3, 2009, 7:55 p.m.
On Tue, Nov 03, 2009 at 07:03:55PM +0100, Eric Dumazet wrote:
> Michael S. Tsirkin a écrit :
> > +static void handle_tx(struct vhost_net *net)
> > +{
> > +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> > +	unsigned head, out, in, s;
> > +	struct msghdr msg = {
> > +		.msg_name = NULL,
> > +		.msg_namelen = 0,
> > +		.msg_control = NULL,
> > +		.msg_controllen = 0,
> > +		.msg_iov = vq->iov,
> > +		.msg_flags = MSG_DONTWAIT,
> > +	};
> > +	size_t len, total_len = 0;
> > +	int err, wmem;
> > +	size_t hdr_size;
> > +	struct socket *sock = rcu_dereference(vq->private_data);
> > +	if (!sock)
> > +		return;
> > +
> > +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> > +	if (wmem >= sock->sk->sk_sndbuf)
> > +		return;
> > +
> > +	use_mm(net->dev.mm);
> > +	mutex_lock(&vq->mutex);
> > +	vhost_no_notify(vq);
> > +
> 
> using rcu_dereference() and mutex_lock() at the same time seems wrong, I suspect
> that your use of RCU is not correct.
> 
> 1) rcu_dereference() should be done inside a read_rcu_lock() section, and
>    we are not allowed to sleep in such a section.
>    (Quoting Documentation/RCU/whatisRCU.txt :
>      It is illegal to block while in an RCU read-side critical section, )
> 
> 2) mutex_lock() can sleep (ie block)

This use is correct. See comment in vhost.h This use of RCU has been
acked by Paul E. McKenney (paulmck@linux.vnet.ibm.com) as well.
There are many ways to use RCU not all of which involve read_rcu_lock.

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Michael S. Tsirkin - Nov. 3, 2009, 7:58 p.m.
On Tue, Nov 03, 2009 at 07:51:35PM +0100, Eric Dumazet wrote:
> Gregory Haskins a écrit :
> > Gregory Haskins wrote:
> >> Eric Dumazet wrote:
> >>> Michael S. Tsirkin a écrit :
> >>>> +static void handle_tx(struct vhost_net *net)
> >>>> +{
> >>>> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> >>>> +	unsigned head, out, in, s;
> >>>> +	struct msghdr msg = {
> >>>> +		.msg_name = NULL,
> >>>> +		.msg_namelen = 0,
> >>>> +		.msg_control = NULL,
> >>>> +		.msg_controllen = 0,
> >>>> +		.msg_iov = vq->iov,
> >>>> +		.msg_flags = MSG_DONTWAIT,
> >>>> +	};
> >>>> +	size_t len, total_len = 0;
> >>>> +	int err, wmem;
> >>>> +	size_t hdr_size;
> >>>> +	struct socket *sock = rcu_dereference(vq->private_data);
> >>>> +	if (!sock)
> >>>> +		return;
> >>>> +
> >>>> +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> >>>> +	if (wmem >= sock->sk->sk_sndbuf)
> >>>> +		return;
> >>>> +
> >>>> +	use_mm(net->dev.mm);
> >>>> +	mutex_lock(&vq->mutex);
> >>>> +	vhost_no_notify(vq);
> >>>> +
> >>> using rcu_dereference() and mutex_lock() at the same time seems wrong, I suspect
> >>> that your use of RCU is not correct.
> >>>
> >>> 1) rcu_dereference() should be done inside a read_rcu_lock() section, and
> >>>    we are not allowed to sleep in such a section.
> >>>    (Quoting Documentation/RCU/whatisRCU.txt :
> >>>      It is illegal to block while in an RCU read-side critical section, )
> >>>
> >>> 2) mutex_lock() can sleep (ie block)
> >>>
> >>
> >> Michael,
> >>   I warned you that this needed better documentation ;)
> >>
> >> Eric,
> >>   I think I flagged this once before, but Michael convinced me that it
> >> was indeed "ok", if but perhaps a bit unconventional.  I will try to
> >> find the thread.
> >>
> >> Kind Regards,
> >> -Greg
> >>
> > 
> > Here it is:
> > 
> > http://lkml.org/lkml/2009/8/12/173
> > 
> 
> Yes, this doesnt convince me at all, and could be a precedent for a wrong RCU use.
> People wanting to use RCU do a grep on kernel sources to find how to correctly
> use RCU.
> 
> Michael, please use existing locking/barrier mechanisms, and not pretend to use RCU.
> 
> Some automatic tools might barf later.
> 
> For example, we could add a debugging facility to check that rcu_dereference() is used
> in an appropriate context, ie conflict with existing mutex_lock() debugging facility.


Paul, you acked this previously. Should I add you acked-by line so
people calm down?  If you would rather I replace
rcu_dereference/rcu_assign_pointer with rmb/wmb, I can do this.
Or maybe patch Documentation to explain this RCU usage?
Eric Dumazet - Nov. 3, 2009, 9:11 p.m.
Michael S. Tsirkin a écrit :
> 
> Paul, you acked this previously. Should I add you acked-by line so
> people calm down?  If you would rather I replace
> rcu_dereference/rcu_assign_pointer with rmb/wmb, I can do this.
> Or maybe patch Documentation to explain this RCU usage?
> 

So you believe I am over-reacting to this dubious use of RCU ?

RCU documentation is already very complex, we dont need to add yet another
subtle use, and makes it less readable.

It seems you use 'RCU api' in drivers/vhost/net.c as convenient macros :

#define rcu_dereference(p)     ({ \
                                typeof(p) _________p1 = ACCESS_ONCE(p); \
                                smp_read_barrier_depends(); \
                                (_________p1); \
                                })

#define rcu_assign_pointer(p, v) \
        ({ \
                if (!__builtin_constant_p(v) || \
                    ((v) != NULL)) \
                        smp_wmb(); \
                (p) = (v); \
        })


There are plenty regular uses of smp_wmb() in kernel, not related to Read Copy Update,
there is nothing wrong to use barriers with appropriate comments.

(And you already use mb(), wmb(), rmb(), smp_wmb() in your patch)


BTW there is at least one locking bug in vhost_net_set_features()

Apparently, mutex_unlock() doesnt trigger a fault if mutex is not locked
by current thread... even with DEBUG_MUTEXES / DEBUG_LOCK_ALLOC


static void vhost_net_set_features(struct vhost_net *n, u64 features)
{
       size_t hdr_size = features & (1 << VHOST_NET_F_VIRTIO_NET_HDR) ?
               sizeof(struct virtio_net_hdr) : 0;
       int i;
<<!>>  mutex_unlock(&n->dev.mutex);
       n->dev.acked_features = features;
       smp_wmb();
       for (i = 0; i < VHOST_NET_VQ_MAX; ++i) {
               mutex_lock(&n->vqs[i].mutex);
               n->vqs[i].hdr_size = hdr_size;
               mutex_unlock(&n->vqs[i].mutex);
       }
       mutex_unlock(&n->dev.mutex);
       vhost_net_flush(n);
}
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Paul E. McKenney - Nov. 3, 2009, 11:57 p.m.
On Tue, Nov 03, 2009 at 01:14:06PM -0500, Gregory Haskins wrote:
> Gregory Haskins wrote:
> > Eric Dumazet wrote:
> >> Michael S. Tsirkin a écrit :
> >>> +static void handle_tx(struct vhost_net *net)
> >>> +{
> >>> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> >>> +	unsigned head, out, in, s;
> >>> +	struct msghdr msg = {
> >>> +		.msg_name = NULL,
> >>> +		.msg_namelen = 0,
> >>> +		.msg_control = NULL,
> >>> +		.msg_controllen = 0,
> >>> +		.msg_iov = vq->iov,
> >>> +		.msg_flags = MSG_DONTWAIT,
> >>> +	};
> >>> +	size_t len, total_len = 0;
> >>> +	int err, wmem;
> >>> +	size_t hdr_size;
> >>> +	struct socket *sock = rcu_dereference(vq->private_data);
> >>> +	if (!sock)
> >>> +		return;
> >>> +
> >>> +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> >>> +	if (wmem >= sock->sk->sk_sndbuf)
> >>> +		return;
> >>> +
> >>> +	use_mm(net->dev.mm);
> >>> +	mutex_lock(&vq->mutex);
> >>> +	vhost_no_notify(vq);
> >>> +
> >> using rcu_dereference() and mutex_lock() at the same time seems wrong, I suspect
> >> that your use of RCU is not correct.
> >>
> >> 1) rcu_dereference() should be done inside a read_rcu_lock() section, and
> >>    we are not allowed to sleep in such a section.
> >>    (Quoting Documentation/RCU/whatisRCU.txt :
> >>      It is illegal to block while in an RCU read-side critical section, )
> >>
> >> 2) mutex_lock() can sleep (ie block)
> >>
> > 
> > 
> > Michael,
> >   I warned you that this needed better documentation ;)
> > 
> > Eric,
> >   I think I flagged this once before, but Michael convinced me that it
> > was indeed "ok", if but perhaps a bit unconventional.  I will try to
> > find the thread.
> > 
> > Kind Regards,
> > -Greg
> > 
> 
> Here it is:
> 
> http://lkml.org/lkml/2009/8/12/173

What was happening in that case was that the rcu_dereference()
was being used in a workqueue item.  The role of rcu_read_lock()
was taken on be the start of execution of the workqueue item, of
rcu_read_unlock() by the end of execution of the workqueue item, and
of synchronize_rcu() by flush_workqueue().  This does work, at least
assuming that flush_workqueue() operates as advertised, which it appears
to at first glance.

The above code looks somewhat different, however -- I don't see
handle_tx() being executed in the context of a work queue.  Instead
it appears to be in an interrupt handler.

So what is the story?  Using synchronize_irq() or some such?

							Thanx, Paul
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Andi Kleen - Nov. 4, 2009, 11:08 a.m.
"Michael S. Tsirkin" <mst@redhat.com> writes:

Haven't really read the whole thing, just noticed something at a glance.

> +/* Expects to be always run from workqueue - which acts as
> + * read-size critical section for our kind of RCU. */
> +static void handle_tx(struct vhost_net *net)
> +{
> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> +	unsigned head, out, in, s;
> +	struct msghdr msg = {
> +		.msg_name = NULL,
> +		.msg_namelen = 0,
> +		.msg_control = NULL,
> +		.msg_controllen = 0,
> +		.msg_iov = vq->iov,
> +		.msg_flags = MSG_DONTWAIT,
> +	};
> +	size_t len, total_len = 0;
> +	int err, wmem;
> +	size_t hdr_size;
> +	struct socket *sock = rcu_dereference(vq->private_data);
> +	if (!sock)
> +		return;
> +
> +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> +	if (wmem >= sock->sk->sk_sndbuf)
> +		return;
> +
> +	use_mm(net->dev.mm);

I haven't gone over all this code in detail, but that isolated reference count
use looks suspicious. What prevents the mm from going away before
you increment, if it's not the current one?

-Andi
Michael S. Tsirkin - Nov. 4, 2009, 11:57 a.m.
On Tue, Nov 03, 2009 at 03:57:44PM -0800, Paul E. McKenney wrote:
> On Tue, Nov 03, 2009 at 01:14:06PM -0500, Gregory Haskins wrote:
> > Gregory Haskins wrote:
> > > Eric Dumazet wrote:
> > >> Michael S. Tsirkin a écrit :
> > >>> +static void handle_tx(struct vhost_net *net)
> > >>> +{
> > >>> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> > >>> +	unsigned head, out, in, s;
> > >>> +	struct msghdr msg = {
> > >>> +		.msg_name = NULL,
> > >>> +		.msg_namelen = 0,
> > >>> +		.msg_control = NULL,
> > >>> +		.msg_controllen = 0,
> > >>> +		.msg_iov = vq->iov,
> > >>> +		.msg_flags = MSG_DONTWAIT,
> > >>> +	};
> > >>> +	size_t len, total_len = 0;
> > >>> +	int err, wmem;
> > >>> +	size_t hdr_size;
> > >>> +	struct socket *sock = rcu_dereference(vq->private_data);
> > >>> +	if (!sock)
> > >>> +		return;
> > >>> +
> > >>> +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> > >>> +	if (wmem >= sock->sk->sk_sndbuf)
> > >>> +		return;
> > >>> +
> > >>> +	use_mm(net->dev.mm);
> > >>> +	mutex_lock(&vq->mutex);
> > >>> +	vhost_no_notify(vq);
> > >>> +
> > >> using rcu_dereference() and mutex_lock() at the same time seems wrong, I suspect
> > >> that your use of RCU is not correct.
> > >>
> > >> 1) rcu_dereference() should be done inside a read_rcu_lock() section, and
> > >>    we are not allowed to sleep in such a section.
> > >>    (Quoting Documentation/RCU/whatisRCU.txt :
> > >>      It is illegal to block while in an RCU read-side critical section, )
> > >>
> > >> 2) mutex_lock() can sleep (ie block)
> > >>
> > > 
> > > 
> > > Michael,
> > >   I warned you that this needed better documentation ;)
> > > 
> > > Eric,
> > >   I think I flagged this once before, but Michael convinced me that it
> > > was indeed "ok", if but perhaps a bit unconventional.  I will try to
> > > find the thread.
> > > 
> > > Kind Regards,
> > > -Greg
> > > 
> > 
> > Here it is:
> > 
> > http://lkml.org/lkml/2009/8/12/173
> 
> What was happening in that case was that the rcu_dereference()
> was being used in a workqueue item.  The role of rcu_read_lock()
> was taken on be the start of execution of the workqueue item, of
> rcu_read_unlock() by the end of execution of the workqueue item, and
> of synchronize_rcu() by flush_workqueue().  This does work, at least
> assuming that flush_workqueue() operates as advertised, which it appears
> to at first glance.
> 
> The above code looks somewhat different, however -- I don't see
> handle_tx() being executed in the context of a work queue.  Instead
> it appears to be in an interrupt handler.
> So what is the story?  Using synchronize_irq() or some such?
> 
> 							Thanx, Paul

No, there has been no change (I won't be able to use a mutex in an
interrupt handler, will I?).  handle_tx is still called in the context
of a work queue: either from handle_tx_kick or from handle_tx_net which
are work queue items.

Can you ack this usage please?
Michael S. Tsirkin - Nov. 4, 2009, 12:04 p.m.
On Tue, Nov 03, 2009 at 10:11:12PM +0100, Eric Dumazet wrote:
> Michael S. Tsirkin a écrit :
> > 
> > Paul, you acked this previously. Should I add you acked-by line so
> > people calm down?  If you would rather I replace
> > rcu_dereference/rcu_assign_pointer with rmb/wmb, I can do this.
> > Or maybe patch Documentation to explain this RCU usage?
> > 
> 
> So you believe I am over-reacting to this dubious use of RCU ?
> 
> RCU documentation is already very complex, we dont need to add yet another
> subtle use, and makes it less readable.
> 
> It seems you use 'RCU api' in drivers/vhost/net.c as convenient macros :
> 
> #define rcu_dereference(p)     ({ \
>                                 typeof(p) _________p1 = ACCESS_ONCE(p); \
>                                 smp_read_barrier_depends(); \
>                                 (_________p1); \
>                                 })
> 
> #define rcu_assign_pointer(p, v) \
>         ({ \
>                 if (!__builtin_constant_p(v) || \
>                     ((v) != NULL)) \
>                         smp_wmb(); \
>                 (p) = (v); \
>         })
> 
> 
> There are plenty regular uses of smp_wmb() in kernel, not related to Read Copy Update,
> there is nothing wrong to use barriers with appropriate comments.

Well, what I do has classic RCU characteristics: readers do not take
locks, writers take a lock and flush after update. This is why I believe
rcu_dereference and rcu_assign_pointer are more appropriate here than
open-coding barriers.

Before deciding whether it's a good idea to open-code barriers
instead, I would like to hear Paul's opinion.

> 
> (And you already use mb(), wmb(), rmb(), smp_wmb() in your patch)

Yes, virtio guest pretty much forces this, there's no way to share
a lock with the guest.

> BTW there is at least one locking bug in vhost_net_set_features()
> 
> Apparently, mutex_unlock() doesnt trigger a fault if mutex is not locked
> by current thread... even with DEBUG_MUTEXES / DEBUG_LOCK_ALLOC
> 
> 
> static void vhost_net_set_features(struct vhost_net *n, u64 features)
> {
>        size_t hdr_size = features & (1 << VHOST_NET_F_VIRTIO_NET_HDR) ?
>                sizeof(struct virtio_net_hdr) : 0;
>        int i;
> <<!>>  mutex_unlock(&n->dev.mutex);
>        n->dev.acked_features = features;
>        smp_wmb();
>        for (i = 0; i < VHOST_NET_VQ_MAX; ++i) {
>                mutex_lock(&n->vqs[i].mutex);
>                n->vqs[i].hdr_size = hdr_size;
>                mutex_unlock(&n->vqs[i].mutex);
>        }
>        mutex_unlock(&n->dev.mutex);
>        vhost_net_flush(n);
> }

Thanks very much for spotting this! Will fix.
Michael S. Tsirkin - Nov. 4, 2009, 12:10 p.m.
On Wed, Nov 04, 2009 at 12:08:47PM +0100, Andi Kleen wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> Haven't really read the whole thing, just noticed something at a glance.
> 
> > +/* Expects to be always run from workqueue - which acts as
> > + * read-size critical section for our kind of RCU. */
> > +static void handle_tx(struct vhost_net *net)
> > +{
> > +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> > +	unsigned head, out, in, s;
> > +	struct msghdr msg = {
> > +		.msg_name = NULL,
> > +		.msg_namelen = 0,
> > +		.msg_control = NULL,
> > +		.msg_controllen = 0,
> > +		.msg_iov = vq->iov,
> > +		.msg_flags = MSG_DONTWAIT,
> > +	};
> > +	size_t len, total_len = 0;
> > +	int err, wmem;
> > +	size_t hdr_size;
> > +	struct socket *sock = rcu_dereference(vq->private_data);
> > +	if (!sock)
> > +		return;
> > +
> > +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> > +	if (wmem >= sock->sk->sk_sndbuf)
> > +		return;
> > +
> > +	use_mm(net->dev.mm);
> 
> I haven't gone over all this code in detail, but that isolated reference count
> use looks suspicious. What prevents the mm from going away before
> you increment, if it's not the current one?

We take a reference to it before we start any virtqueues,
and stop all virtqueues before we drop the reference:
/* Caller should have device mutex */
static long vhost_dev_set_owner(struct vhost_dev *dev)
{
        /* Is there an owner already? */
        if (dev->mm)
                return -EBUSY;
        /* No owner, become one */
        dev->mm = get_task_mm(current);
        return 0;
}

And 
vhost_dev_cleanup:
....

        if (dev->mm)
                mmput(dev->mm);
        dev->mm = NULL;
}


Fine?

> -Andi 
> 
> -- 
> ak@linux.intel.com -- Speaking for myself only.
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Andi Kleen - Nov. 4, 2009, 12:59 p.m.
> Fine?

I cannot say -- are there paths that could drop the device beforehand?
(as in do you hold a reference to it?)

-Andi
Michael S. Tsirkin - Nov. 4, 2009, 1:08 p.m.
On Wed, Nov 04, 2009 at 01:59:57PM +0100, Andi Kleen wrote:
> > Fine?
> 
> I cannot say -- are there paths that could drop the device beforehand?

Do you mean drop the mm reference?

> (as in do you hold a reference to it?)

By design I think I always have a reference to mm before I use it.

This works like this:
ioctl SET_OWNER - calls get_task_mm, I think this gets a reference to mm
ioctl SET_BACKEND - checks that SET_OWNER was run, starts virtqueue
ioctl RESET_OWNER - stops virtqueues, drops the reference to mm
file close - stops virtqueues, if we still have it then drops mm

This is why I think I can call use_mm/unuse_mm while virtqueue is running,
safely.
Makes sense?

> -Andi
> -- 
> ak@linux.intel.com -- Speaking for myself only.
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Andi Kleen - Nov. 4, 2009, 1:15 p.m.
On Wed, Nov 04, 2009 at 03:08:28PM +0200, Michael S. Tsirkin wrote:
> On Wed, Nov 04, 2009 at 01:59:57PM +0100, Andi Kleen wrote:
> > > Fine?
> > 
> > I cannot say -- are there paths that could drop the device beforehand?
> 
> Do you mean drop the mm reference?

No the reference to the device, which owns the mm for you.

> 
> > (as in do you hold a reference to it?)
> 
> By design I think I always have a reference to mm before I use it.
> 
> This works like this:
> ioctl SET_OWNER - calls get_task_mm, I think this gets a reference to mm
> ioctl SET_BACKEND - checks that SET_OWNER was run, starts virtqueue
> ioctl RESET_OWNER - stops virtqueues, drops the reference to mm
> file close - stops virtqueues, if we still have it then drops mm
> 
> This is why I think I can call use_mm/unuse_mm while virtqueue is running,
> safely.
> Makes sense?

Do you protect against another thread doing RESET_OWNER in parallel while
RESET_OWNER runs?

-Andi
Michael S. Tsirkin - Nov. 4, 2009, 1:17 p.m.
On Wed, Nov 04, 2009 at 02:15:33PM +0100, Andi Kleen wrote:
> On Wed, Nov 04, 2009 at 03:08:28PM +0200, Michael S. Tsirkin wrote:
> > On Wed, Nov 04, 2009 at 01:59:57PM +0100, Andi Kleen wrote:
> > > > Fine?
> > > 
> > > I cannot say -- are there paths that could drop the device beforehand?
> > 
> > Do you mean drop the mm reference?
> 
> No the reference to the device, which owns the mm for you.

The device is created when file is open and destroyed
when file is closed. So I think the fs code handles the
reference counting for me: it won't call file cleanup
callback while some userspace process has the file open.
Right?

> > 
> > > (as in do you hold a reference to it?)
> > 
> > By design I think I always have a reference to mm before I use it.
> > 
> > This works like this:
> > ioctl SET_OWNER - calls get_task_mm, I think this gets a reference to mm
> > ioctl SET_BACKEND - checks that SET_OWNER was run, starts virtqueue
> > ioctl RESET_OWNER - stops virtqueues, drops the reference to mm
> > file close - stops virtqueues, if we still have it then drops mm
> > 
> > This is why I think I can call use_mm/unuse_mm while virtqueue is running,
> > safely.
> > Makes sense?
> 
> Do you protect against another thread doing RESET_OWNER in parallel while
> RESET_OWNER runs?

Yes, I have a mutex in the device for that. Same with SET_BACKEND.

> -Andi
> 
> -- 
> ak@linux.intel.com -- Speaking for myself only.
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Andi Kleen - Nov. 4, 2009, 1:37 p.m.
On Wed, Nov 04, 2009 at 03:17:36PM +0200, Michael S. Tsirkin wrote:
> On Wed, Nov 04, 2009 at 02:15:33PM +0100, Andi Kleen wrote:
> > On Wed, Nov 04, 2009 at 03:08:28PM +0200, Michael S. Tsirkin wrote:
> > > On Wed, Nov 04, 2009 at 01:59:57PM +0100, Andi Kleen wrote:
> > > > > Fine?
> > > > 
> > > > I cannot say -- are there paths that could drop the device beforehand?
> > > 
> > > Do you mean drop the mm reference?
> > 
> > No the reference to the device, which owns the mm for you.
> 
> The device is created when file is open and destroyed
> when file is closed. So I think the fs code handles the
> reference counting for me: it won't call file cleanup
> callback while some userspace process has the file open.
> Right?

Yes.

But the semantics when someone inherits such a fd through exec
or through file descriptor passing would be surely "interesting"
You would still do IO on the old VM.

I guess it would be a good way to confuse memory accounting schemes 
or administrators @)

It would be all saner if this was all a single atomic step.

-Andi
Michael S. Tsirkin - Nov. 4, 2009, 1:41 p.m.
On Wed, Nov 04, 2009 at 02:37:28PM +0100, Andi Kleen wrote:
> On Wed, Nov 04, 2009 at 03:17:36PM +0200, Michael S. Tsirkin wrote:
> > On Wed, Nov 04, 2009 at 02:15:33PM +0100, Andi Kleen wrote:
> > > On Wed, Nov 04, 2009 at 03:08:28PM +0200, Michael S. Tsirkin wrote:
> > > > On Wed, Nov 04, 2009 at 01:59:57PM +0100, Andi Kleen wrote:
> > > > > > Fine?
> > > > > 
> > > > > I cannot say -- are there paths that could drop the device beforehand?
> > > > 
> > > > Do you mean drop the mm reference?
> > > 
> > > No the reference to the device, which owns the mm for you.
> > 
> > The device is created when file is open and destroyed
> > when file is closed. So I think the fs code handles the
> > reference counting for me: it won't call file cleanup
> > callback while some userspace process has the file open.
> > Right?
> 
> Yes.
> 
> But the semantics when someone inherits such a fd through exec
> or through file descriptor passing would be surely "interesting"
> You would still do IO on the old VM.
> 
> I guess it would be a good way to confuse memory accounting schemes 
> or administrators @)
> It would be all saner if this was all a single atomic step.
> 
> -Andi

I have this atomic actually. A child process will first thing
do SET_OWNER: this is required before any other operation.

SET_OWNER atomically (under mutex) does two things:
- check that there is no other owner
- get mm and set current process as owner

I hope this addresses your concern?
Michael S. Tsirkin - Nov. 4, 2009, 4:37 p.m.
On Wed, Nov 04, 2009 at 03:41:47PM +0200, Michael S. Tsirkin wrote:
> On Wed, Nov 04, 2009 at 02:37:28PM +0100, Andi Kleen wrote:
> > On Wed, Nov 04, 2009 at 03:17:36PM +0200, Michael S. Tsirkin wrote:
> > > On Wed, Nov 04, 2009 at 02:15:33PM +0100, Andi Kleen wrote:
> > > > On Wed, Nov 04, 2009 at 03:08:28PM +0200, Michael S. Tsirkin wrote:
> > > > > On Wed, Nov 04, 2009 at 01:59:57PM +0100, Andi Kleen wrote:
> > > > > > > Fine?
> > > > > > 
> > > > > > I cannot say -- are there paths that could drop the device beforehand?
> > > > > 
> > > > > Do you mean drop the mm reference?
> > > > 
> > > > No the reference to the device, which owns the mm for you.
> > > 
> > > The device is created when file is open and destroyed
> > > when file is closed. So I think the fs code handles the
> > > reference counting for me: it won't call file cleanup
> > > callback while some userspace process has the file open.
> > > Right?
> > 
> > Yes.
> > 
> > But the semantics when someone inherits such a fd through exec
> > or through file descriptor passing would be surely "interesting"
> > You would still do IO on the old VM.
> > 
> > I guess it would be a good way to confuse memory accounting schemes 
> > or administrators @)
> > It would be all saner if this was all a single atomic step.
> > 
> > -Andi
> 
> I have this atomic actually. A child process will first thing
> do SET_OWNER: this is required before any other operation.
> 
> SET_OWNER atomically (under mutex) does two things:
> - check that there is no other owner
> - get mm and set current process as owner
> 
> I hope this addresses your concern?

Andrea, since you looked at this design at the early stages,
maybe you can provide feedback on the following question:

vhost has an ioctl to do get_task_mm and store the mm in per-file device
structure.  mmput is called when file is closed.  vhost is careful not
to reference the mm after is has been put. There is also an atomic
mutual exclusion mechanism to ensure that vhost does not allow one
process to access another's mm, even if they share a vhost file
descriptor.  But, this still means that mm structure can outlive the
task if the file descriptor is shared with another process.

Other drivers, such as kvm, have the same property.

Do you think this is OK?

Thanks!
Paul E. McKenney - Nov. 4, 2009, 5:25 p.m.
On Wed, Nov 04, 2009 at 01:57:29PM +0200, Michael S. Tsirkin wrote:
> On Tue, Nov 03, 2009 at 03:57:44PM -0800, Paul E. McKenney wrote:
> > On Tue, Nov 03, 2009 at 01:14:06PM -0500, Gregory Haskins wrote:
> > > Gregory Haskins wrote:
> > > > Eric Dumazet wrote:
> > > >> Michael S. Tsirkin a écrit :
> > > >>> +static void handle_tx(struct vhost_net *net)
> > > >>> +{
> > > >>> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> > > >>> +	unsigned head, out, in, s;
> > > >>> +	struct msghdr msg = {
> > > >>> +		.msg_name = NULL,
> > > >>> +		.msg_namelen = 0,
> > > >>> +		.msg_control = NULL,
> > > >>> +		.msg_controllen = 0,
> > > >>> +		.msg_iov = vq->iov,
> > > >>> +		.msg_flags = MSG_DONTWAIT,
> > > >>> +	};
> > > >>> +	size_t len, total_len = 0;
> > > >>> +	int err, wmem;
> > > >>> +	size_t hdr_size;
> > > >>> +	struct socket *sock = rcu_dereference(vq->private_data);
> > > >>> +	if (!sock)
> > > >>> +		return;
> > > >>> +
> > > >>> +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> > > >>> +	if (wmem >= sock->sk->sk_sndbuf)
> > > >>> +		return;
> > > >>> +
> > > >>> +	use_mm(net->dev.mm);
> > > >>> +	mutex_lock(&vq->mutex);
> > > >>> +	vhost_no_notify(vq);
> > > >>> +
> > > >> using rcu_dereference() and mutex_lock() at the same time seems wrong, I suspect
> > > >> that your use of RCU is not correct.
> > > >>
> > > >> 1) rcu_dereference() should be done inside a read_rcu_lock() section, and
> > > >>    we are not allowed to sleep in such a section.
> > > >>    (Quoting Documentation/RCU/whatisRCU.txt :
> > > >>      It is illegal to block while in an RCU read-side critical section, )
> > > >>
> > > >> 2) mutex_lock() can sleep (ie block)
> > > >>
> > > > 
> > > > 
> > > > Michael,
> > > >   I warned you that this needed better documentation ;)
> > > > 
> > > > Eric,
> > > >   I think I flagged this once before, but Michael convinced me that it
> > > > was indeed "ok", if but perhaps a bit unconventional.  I will try to
> > > > find the thread.
> > > > 
> > > > Kind Regards,
> > > > -Greg
> > > > 
> > > 
> > > Here it is:
> > > 
> > > http://lkml.org/lkml/2009/8/12/173
> > 
> > What was happening in that case was that the rcu_dereference()
> > was being used in a workqueue item.  The role of rcu_read_lock()
> > was taken on be the start of execution of the workqueue item, of
> > rcu_read_unlock() by the end of execution of the workqueue item, and
> > of synchronize_rcu() by flush_workqueue().  This does work, at least
> > assuming that flush_workqueue() operates as advertised, which it appears
> > to at first glance.
> > 
> > The above code looks somewhat different, however -- I don't see
> > handle_tx() being executed in the context of a work queue.  Instead
> > it appears to be in an interrupt handler.
> > So what is the story?  Using synchronize_irq() or some such?
> > 
> > 							Thanx, Paul
> 
> No, there has been no change (I won't be able to use a mutex in an
> interrupt handler, will I?).  handle_tx is still called in the context
> of a work queue: either from handle_tx_kick or from handle_tx_net which
> are work queue items.

Ah, my mistake -- I was looking at 2.6.31 rather than latest git with
your patches.

> Can you ack this usage please?

I thought I had done so in my paragraph above, but if you would like
something a bit more formal...

	I, Paul E. McKenney, maintainer of the RCU implmentation
	embodied in the Linux kernel and co-inventor of RCU, being of
	sound mind and body, notwithstanding the wear and tear inherent
	in my numerous decades sojourn on this planet, hereby declare
	that the following usage of work queues constitutes a valid
	RCU implementation:

	1.	Execution of a full workqueue item being substituted
		for a conventional RCU read-side critical section, so
		that the start of execution of the function specified to
		INIT_WORK() corresponds to rcu_read_lock(), and the end of
		this self-same function corresponds to rcu_read_unlock().

	2.	Execution of flush_workqueue() being substituted for
		the conventional synchronize_rcu().

	The kernel developer availing himself or herself of this
	declaration must observe the following caveats:

	a.	The function specified to INIT_WORK() may only be
		invoked via the workqueue mechanism.  Invoking said
		function directly renders this declaration null
		and void, as it prevents the flush_workqueue() function
		from delivering the fundamental guarantee inherent in RCU.

	b.	At some point in the future, said developer may be
		required to apply some gcc attribute or sparse annotation
		to the function passed to INIT_WORK().	Beyond that
		point, failure to comply will render this declaration
		null and void, as such failure would render inoperative
		some potential RCU-validation tools, as duly noted by
		Eric Dumazet.

	c.	This declaration in no way relieves the developer of
		the responsibility to use this and other synchronization
		mechanisms correctly, again, as duly noted by Eric
		Dumazet.

(Sorry, but, as always, I could not resist!)

							Thanx, Paul
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Eric Dumazet - Nov. 4, 2009, 5:33 p.m.
Paul E. McKenney a écrit :
> 
> (Sorry, but, as always, I could not resist!)

Yesssss :)

Thanks Paul for this masterpiece of diplomatic "Acked-by" ;)



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Michael S. Tsirkin - Nov. 4, 2009, 7:06 p.m.
On Wed, Nov 04, 2009 at 09:25:42AM -0800, Paul E. McKenney wrote:
> (Sorry, but, as always, I could not resist!)
> 
> 							Thanx, Paul

Thanks Paul!
Jonathan: are you reading this?
Another one for your quotes of the week collection :)
Gregory Haskins - Nov. 4, 2009, 7:12 p.m.
Michael S. Tsirkin wrote:
> On Wed, Nov 04, 2009 at 09:25:42AM -0800, Paul E. McKenney wrote:
>> (Sorry, but, as always, I could not resist!)
>>
>> 							Thanx, Paul
> 
> Thanks Paul!
> Jonathan: are you reading this?
> Another one for your quotes of the week collection :)
> 

I second that.
Rusty Russell - Nov. 6, 2009, 5:01 a.m.
On Thu, 5 Nov 2009 03:55:42 am Paul E. McKenney wrote:
> On Wed, Nov 04, 2009 at 01:57:29PM +0200, Michael S. Tsirkin wrote:
> > Can you ack this usage please?
> 
> I thought I had done so in my paragraph above, but if you would like
> something a bit more formal...

<snip verbose super-ack with qualifications>

That's great guys.  And yes, this is a kind of read-copy-update.  And no,
there's nothing wrong with it.

But it's still nasty to use half an API.  If it were a few places I would
have open-coded it with a comment, or wrapped it.  As it is, I don't think
that would be a win.

Cheers,
Rusty.
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Paul E. McKenney - Nov. 6, 2009, 4:30 p.m.
On Fri, Nov 06, 2009 at 03:31:20PM +1030, Rusty Russell wrote:
> On Thu, 5 Nov 2009 03:55:42 am Paul E. McKenney wrote:
> > On Wed, Nov 04, 2009 at 01:57:29PM +0200, Michael S. Tsirkin wrote:
> > > Can you ack this usage please?
> > 
> > I thought I had done so in my paragraph above, but if you would like
> > something a bit more formal...
> 
> <snip verbose super-ack with qualifications>
> 
> That's great guys.  And yes, this is a kind of read-copy-update.  And no,
> there's nothing wrong with it.
> 
> But it's still nasty to use half an API.  If it were a few places I would
> have open-coded it with a comment, or wrapped it.  As it is, I don't think
> that would be a win.

So would it help to have a rcu_read_lock_workqueue() and
rcu_read_unlock_workqueue() that checked nesting and whether they were
actually running in the context of a workqueue item?  Or did you have
something else in mind?  Or am I misjudging the level of sarcasm in
your reply?  ;-)

							Thanx, Paul
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Rusty Russell - Nov. 8, 2009, 4:09 a.m.
On Sat, 7 Nov 2009 03:00:07 am Paul E. McKenney wrote:
> On Fri, Nov 06, 2009 at 03:31:20PM +1030, Rusty Russell wrote:
> > But it's still nasty to use half an API.  If it were a few places I would
> > have open-coded it with a comment, or wrapped it.  As it is, I don't think
> > that would be a win.
> 
> So would it help to have a rcu_read_lock_workqueue() and
> rcu_read_unlock_workqueue() that checked nesting and whether they were
> actually running in the context of a workqueue item?  Or did you have
> something else in mind?  Or am I misjudging the level of sarcasm in
> your reply?  ;-)

You read correctly.  If we get a second user, creating an API makes sense.

Thanks,
Rusty.
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Paul E. McKenney - Nov. 8, 2009, 7:36 p.m.
On Sun, Nov 08, 2009 at 02:39:59PM +1030, Rusty Russell wrote:
> On Sat, 7 Nov 2009 03:00:07 am Paul E. McKenney wrote:
> > On Fri, Nov 06, 2009 at 03:31:20PM +1030, Rusty Russell wrote:
> > > But it's still nasty to use half an API.  If it were a few places I would
> > > have open-coded it with a comment, or wrapped it.  As it is, I don't think
> > > that would be a win.
> > 
> > So would it help to have a rcu_read_lock_workqueue() and
> > rcu_read_unlock_workqueue() that checked nesting and whether they were
> > actually running in the context of a workqueue item?  Or did you have
> > something else in mind?  Or am I misjudging the level of sarcasm in
> > your reply?  ;-)
> 
> You read correctly.  If we get a second user, creating an API makes sense.

Makes sense to me as well.  Which does provide some time to come up with
a primitive designed to answer the question "Am I currently executing in
the context of a workqueue item?".  ;-)

							Thanx, Paul
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Patch

diff --git a/MAINTAINERS b/MAINTAINERS
index 8824115..980a69b 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -5619,6 +5619,15 @@  S:	Maintained
 F:	Documentation/filesystems/vfat.txt
 F:	fs/fat/
 
+VIRTIO HOST (VHOST)
+M:	"Michael S. Tsirkin" <mst@redhat.com>
+L:	kvm@vger.kernel.org
+L:	virtualization@lists.osdl.org
+L:	netdev@vger.kernel.org
+S:	Maintained
+F:	drivers/vhost/
+F:	include/linux/vhost.h
+
 VIA RHINE NETWORK DRIVER
 M:	Roger Luethi <rl@hellgate.ch>
 S:	Maintained
diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index b84e571..94f44d9 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -64,6 +64,7 @@  config KVM_AMD
 
 # OK, it's a little counter-intuitive to do this, but it puts it neatly under
 # the virtualization menu.
+source drivers/vhost/Kconfig
 source drivers/lguest/Kconfig
 source drivers/virtio/Kconfig
 
diff --git a/drivers/Makefile b/drivers/Makefile
index 6ee53c7..81e3659 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -106,6 +106,7 @@  obj-$(CONFIG_HID)		+= hid/
 obj-$(CONFIG_PPC_PS3)		+= ps3/
 obj-$(CONFIG_OF)		+= of/
 obj-$(CONFIG_SSB)		+= ssb/
+obj-$(CONFIG_VHOST_NET)		+= vhost/
 obj-$(CONFIG_VIRTIO)		+= virtio/
 obj-$(CONFIG_VLYNQ)		+= vlynq/
 obj-$(CONFIG_STAGING)		+= staging/
diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
new file mode 100644
index 0000000..d955406
--- /dev/null
+++ b/drivers/vhost/Kconfig
@@ -0,0 +1,11 @@ 
+config VHOST_NET
+	tristate "Host kernel accelerator for virtio net"
+	depends on NET && EVENTFD
+	---help---
+	  This kernel module can be loaded in host kernel to accelerate
+	  guest networking with virtio_net. Not to be confused with virtio_net
+	  module itself which needs to be loaded in guest kernel.
+
+	  To compile this driver as a module, choose M here: the module will
+	  be called vhost_net.
+
diff --git a/drivers/vhost/Makefile b/drivers/vhost/Makefile
new file mode 100644
index 0000000..72dd020
--- /dev/null
+++ b/drivers/vhost/Makefile
@@ -0,0 +1,2 @@ 
+obj-$(CONFIG_VHOST_NET) += vhost_net.o
+vhost_net-y := vhost.o net.o
diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
new file mode 100644
index 0000000..4af3b98
--- /dev/null
+++ b/drivers/vhost/net.c
@@ -0,0 +1,633 @@ 
+/* Copyright (C) 2009 Red Hat, Inc.
+ * Author: Michael S. Tsirkin <mst@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.
+ *
+ * virtio-net server in host kernel.
+ */
+
+#include <linux/compat.h>
+#include <linux/eventfd.h>
+#include <linux/vhost.h>
+#include <linux/virtio_net.h>
+#include <linux/mmu_context.h>
+#include <linux/miscdevice.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/workqueue.h>
+#include <linux/rcupdate.h>
+#include <linux/file.h>
+
+#include <linux/net.h>
+#include <linux/if_packet.h>
+#include <linux/if_arp.h>
+#include <linux/if_tun.h>
+
+#include <net/sock.h>
+
+#include "vhost.h"
+
+/* Max number of bytes transferred before requeueing the job.
+ * Using this limit prevents one virtqueue from starving others. */
+#define VHOST_NET_WEIGHT 0x80000
+
+enum {
+	VHOST_NET_VQ_RX = 0,
+	VHOST_NET_VQ_TX = 1,
+	VHOST_NET_VQ_MAX = 2,
+};
+
+enum vhost_net_poll_state {
+	VHOST_NET_POLL_DISABLED = 0,
+	VHOST_NET_POLL_STARTED = 1,
+	VHOST_NET_POLL_STOPPED = 2,
+};
+
+struct vhost_net {
+	struct vhost_dev dev;
+	struct vhost_virtqueue vqs[VHOST_NET_VQ_MAX];
+	struct vhost_poll poll[VHOST_NET_VQ_MAX];
+	/* Tells us whether we are polling a socket for TX.
+	 * We only do this when socket buffer fills up.
+	 * Protected by tx vq lock. */
+	enum vhost_net_poll_state tx_poll_state;
+};
+
+/* Pop first len bytes from iovec. Return number of segments used. */
+static int move_iovec_hdr(struct iovec *from, struct iovec *to,
+			  size_t len, int iov_count)
+{
+	int seg = 0;
+	size_t size;
+	while (len && seg < iov_count) {
+		size = min(from->iov_len, len);
+		to->iov_base = from->iov_base;
+		to->iov_len = size;
+		from->iov_len -= size;
+		from->iov_base += size;
+		len -= size;
+		++from;
+		++to;
+		++seg;
+	}
+	return seg;
+}
+
+/* Caller must have TX VQ lock */
+static void tx_poll_stop(struct vhost_net *net)
+{
+	if (likely(net->tx_poll_state != VHOST_NET_POLL_STARTED))
+		return;
+	vhost_poll_stop(net->poll + VHOST_NET_VQ_TX);
+	net->tx_poll_state = VHOST_NET_POLL_STOPPED;
+}
+
+/* Caller must have TX VQ lock */
+static void tx_poll_start(struct vhost_net *net, struct socket *sock)
+{
+	if (unlikely(net->tx_poll_state != VHOST_NET_POLL_STOPPED))
+		return;
+	vhost_poll_start(net->poll + VHOST_NET_VQ_TX, sock->file);
+	net->tx_poll_state = VHOST_NET_POLL_STARTED;
+}
+
+/* Expects to be always run from workqueue - which acts as
+ * read-size critical section for our kind of RCU. */
+static void handle_tx(struct vhost_net *net)
+{
+	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
+	unsigned head, out, in, s;
+	struct msghdr msg = {
+		.msg_name = NULL,
+		.msg_namelen = 0,
+		.msg_control = NULL,
+		.msg_controllen = 0,
+		.msg_iov = vq->iov,
+		.msg_flags = MSG_DONTWAIT,
+	};
+	size_t len, total_len = 0;
+	int err, wmem;
+	size_t hdr_size;
+	struct socket *sock = rcu_dereference(vq->private_data);
+	if (!sock)
+		return;
+
+	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
+	if (wmem >= sock->sk->sk_sndbuf)
+		return;
+
+	use_mm(net->dev.mm);
+	mutex_lock(&vq->mutex);
+	vhost_no_notify(vq);
+
+	if (wmem < sock->sk->sk_sndbuf * 2)
+		tx_poll_stop(net);
+	hdr_size = vq->hdr_size;
+
+	for (;;) {
+		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in,
+					 NULL, NULL);
+		/* Nothing new?  Wait for eventfd to tell us they refilled. */
+		if (head == vq->num) {
+			wmem = atomic_read(&sock->sk->sk_wmem_alloc);
+			if (wmem >= sock->sk->sk_sndbuf * 3 / 4) {
+				tx_poll_start(net, sock);
+				set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
+				break;
+			}
+			if (vhost_notify(vq))
+				continue;
+			break;
+		}
+		if (in) {
+			vq_err(vq, "Unexpected descriptor format for TX: "
+			       "out %d, int %d\n", out, in);
+			break;
+		}
+		/* Skip header. TODO: support TSO. */
+		s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, out);
+		msg.msg_iovlen = out;
+		len = iov_length(vq->iov, out);
+		/* Sanity check */
+		if (!len) {
+			vq_err(vq, "Unexpected header len for TX: "
+			       "%ld expected %zd\n",
+			       iov_length(vq->hdr, s), hdr_size);
+			break;
+		}
+		/* TODO: Check specific error and bomb out unless ENOBUFS? */
+		err = sock->ops->sendmsg(NULL, sock, &msg, len);
+		if (unlikely(err < 0)) {
+			vhost_discard_vq_desc(vq);
+			tx_poll_start(net, sock);
+			break;
+		}
+		if (err != len)
+			pr_err("Truncated TX packet: "
+			       " len %d != %zd\n", err, len);
+		vhost_add_used_and_trigger(&net->dev, vq, head, 0);
+		total_len += len;
+		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
+			vhost_poll_queue(&vq->poll);
+			break;
+		}
+	}
+
+	mutex_unlock(&vq->mutex);
+	unuse_mm(net->dev.mm);
+}
+
+/* Expects to be always run from workqueue - which acts as
+ * read-size critical section for our kind of RCU. */
+static void handle_rx(struct vhost_net *net)
+{
+	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_RX];
+	unsigned head, out, in, log, s;
+	struct vhost_log *vq_log;
+	struct msghdr msg = {
+		.msg_name = NULL,
+		.msg_namelen = 0,
+		.msg_control = NULL, /* FIXME: get and handle RX aux data. */
+		.msg_controllen = 0,
+		.msg_iov = vq->iov,
+		.msg_flags = MSG_DONTWAIT,
+	};
+
+	struct virtio_net_hdr hdr = {
+		.flags = 0,
+		.gso_type = VIRTIO_NET_HDR_GSO_NONE
+	};
+
+	size_t len, total_len = 0;
+	int err;
+	size_t hdr_size;
+	struct socket *sock = rcu_dereference(vq->private_data);
+	if (!sock || skb_queue_empty(&sock->sk->sk_receive_queue))
+		return;
+
+	use_mm(net->dev.mm);
+	mutex_lock(&vq->mutex);
+	vhost_no_notify(vq);
+	hdr_size = vq->hdr_size;
+
+	vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ?
+		vq->log : NULL;
+
+	for (;;) {
+		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in,
+					 vq_log, &log);
+		/* OK, now we need to know about added descriptors. */
+		if (head == vq->num && vhost_notify(vq))
+			/* They could have slipped one in as we were doing that:
+			 * check again. */
+			continue;
+		/* Nothing new?  Wait for eventfd to tell us they refilled. */
+		if (head == vq->num)
+			break;
+		/* We don't need to be notified again. */
+		vhost_no_notify(vq);
+		if (out) {
+			vq_err(vq, "Unexpected descriptor format for RX: "
+			       "out %d, int %d\n",
+			       out, in);
+			break;
+		}
+		/* Skip header. TODO: support TSO/mergeable rx buffers. */
+		s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, in);
+		msg.msg_iovlen = in;
+		len = iov_length(vq->iov, in);
+		/* Sanity check */
+		if (!len) {
+			vq_err(vq, "Unexpected header len for RX: "
+			       "%zd expected %zd\n",
+			       iov_length(vq->hdr, s), hdr_size);
+			break;
+		}
+		err = sock->ops->recvmsg(NULL, sock, &msg,
+					 len, MSG_DONTWAIT | MSG_TRUNC);
+		/* TODO: Check specific error and bomb out unless EAGAIN? */
+		if (err < 0) {
+			vhost_discard_vq_desc(vq);
+			break;
+		}
+		/* TODO: Should check and handle checksum. */
+		if (err > len) {
+			pr_err("Discarded truncated rx packet: "
+			       " len %d > %zd\n", err, len);
+			vhost_discard_vq_desc(vq);
+			continue;
+		}
+		len = err;
+		err = memcpy_toiovec(vq->hdr, (unsigned char *)&hdr, hdr_size);
+		if (err) {
+			vq_err(vq, "Unable to write vnet_hdr at addr %p: %d\n",
+			       vq->iov->iov_base, err);
+			break;
+		}
+		len += hdr_size;
+		vhost_add_used_and_trigger(&net->dev, vq, head, len);
+		if (unlikely(vq_log))
+			vhost_log_write(vq, vq_log, log, len);
+		total_len += len;
+		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
+			vhost_poll_queue(&vq->poll);
+			break;
+		}
+	}
+
+	mutex_unlock(&vq->mutex);
+	unuse_mm(net->dev.mm);
+}
+
+static void handle_tx_kick(struct work_struct *work)
+{
+	struct vhost_virtqueue *vq;
+	struct vhost_net *net;
+	vq = container_of(work, struct vhost_virtqueue, poll.work);
+	net = container_of(vq->dev, struct vhost_net, dev);
+	handle_tx(net);
+}
+
+static void handle_rx_kick(struct work_struct *work)
+{
+	struct vhost_virtqueue *vq;
+	struct vhost_net *net;
+	vq = container_of(work, struct vhost_virtqueue, poll.work);
+	net = container_of(vq->dev, struct vhost_net, dev);
+	handle_rx(net);
+}
+
+static void handle_tx_net(struct work_struct *work)
+{
+	struct vhost_net *net;
+	net = container_of(work, struct vhost_net, poll[VHOST_NET_VQ_TX].work);
+	handle_tx(net);
+}
+
+static void handle_rx_net(struct work_struct *work)
+{
+	struct vhost_net *net;
+	net = container_of(work, struct vhost_net, poll[VHOST_NET_VQ_RX].work);
+	handle_rx(net);
+}
+
+static int vhost_net_open(struct inode *inode, struct file *f)
+{
+	struct vhost_net *n = kzalloc(sizeof *n, GFP_KERNEL);
+	int r;
+	if (!n)
+		return -ENOMEM;
+	f->private_data = n;
+	n->vqs[VHOST_NET_VQ_TX].handle_kick = handle_tx_kick;
+	n->vqs[VHOST_NET_VQ_RX].handle_kick = handle_rx_kick;
+	r = vhost_dev_init(&n->dev, n->vqs, VHOST_NET_VQ_MAX);
+	if (r < 0) {
+		kfree(n);
+		return r;
+	}
+
+	vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT);
+	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN);
+	n->tx_poll_state = VHOST_NET_POLL_DISABLED;
+	return 0;
+}
+
+static void vhost_net_disable_vq(struct vhost_net *n, int index)
+{
+	if (!n->vqs[index].private_data)
+		return;
+	if (index == VHOST_NET_VQ_TX) {
+		tx_poll_stop(n);
+		n->tx_poll_state = VHOST_NET_POLL_DISABLED;
+	} else
+		vhost_poll_stop(n->poll + VHOST_NET_VQ_RX);
+}
+
+static void vhost_net_enable_vq(struct vhost_net *n, int index)
+{
+	struct socket *sock = n->vqs[index].private_data;
+	if (!sock)
+		return;
+	if (index == VHOST_NET_VQ_TX) {
+		n->tx_poll_state = VHOST_NET_POLL_STOPPED;
+		tx_poll_start(n, sock);
+	} else
+		vhost_poll_start(n->poll + VHOST_NET_VQ_RX, sock->file);
+}
+
+static struct socket *vhost_net_stop_vq(struct vhost_net *n, int index)
+{
+	struct socket *sock;
+
+	mutex_lock(&n->vqs[index].mutex);
+	sock = n->vqs[index].private_data;
+	vhost_net_disable_vq(n, index);
+	rcu_assign_pointer(n->vqs[index].private_data, NULL);
+	mutex_unlock(&n->vqs[index].mutex);
+	return sock;
+}
+
+static void vhost_net_stop(struct vhost_net *n, struct socket **tx_sock,
+			   struct socket **rx_sock)
+{
+	*tx_sock = vhost_net_stop_vq(n, VHOST_NET_VQ_TX);
+	*rx_sock = vhost_net_stop_vq(n, VHOST_NET_VQ_RX);
+}
+
+static void vhost_net_flush_vq(struct vhost_net *n, int index)
+{
+	vhost_poll_flush(n->poll + index);
+	vhost_poll_flush(&n->dev.vqs[index].poll);
+}
+
+static void vhost_net_flush(struct vhost_net *n)
+{
+	vhost_net_flush_vq(n, VHOST_NET_VQ_TX);
+	vhost_net_flush_vq(n, VHOST_NET_VQ_RX);
+}
+
+static int vhost_net_release(struct inode *inode, struct file *f)
+{
+	struct vhost_net *n = f->private_data;
+	struct socket *tx_sock;
+	struct socket *rx_sock;
+
+	vhost_net_stop(n, &tx_sock, &rx_sock);
+	vhost_net_flush(n);
+	vhost_dev_cleanup(&n->dev);
+	if (tx_sock)
+		fput(tx_sock->file);
+	if (rx_sock)
+		fput(rx_sock->file);
+	/* We do an extra flush before freeing memory,
+	 * since jobs can re-queue themselves. */
+	vhost_net_flush(n);
+	kfree(n);
+	return 0;
+}
+
+static struct socket *get_raw_socket(int fd)
+{
+	struct {
+		struct sockaddr_ll sa;
+		char  buf[MAX_ADDR_LEN];
+	} uaddr;
+	int uaddr_len = sizeof uaddr, r;
+	struct socket *sock = sockfd_lookup(fd, &r);
+	if (!sock)
+		return ERR_PTR(-ENOTSOCK);
+
+	/* Parameter checking */
+	if (sock->sk->sk_type != SOCK_RAW) {
+		r = -ESOCKTNOSUPPORT;
+		goto err;
+	}
+
+	r = sock->ops->getname(sock, (struct sockaddr *)&uaddr.sa,
+			       &uaddr_len, 0);
+	if (r)
+		goto err;
+
+	if (uaddr.sa.sll_family != AF_PACKET) {
+		r = -EPFNOSUPPORT;
+		goto err;
+	}
+	return sock;
+err:
+	fput(sock->file);
+	return ERR_PTR(r);
+}
+
+static struct socket *get_tun_socket(int fd)
+{
+	struct file *file = fget(fd);
+	struct socket *sock;
+	if (!file)
+		return ERR_PTR(-EBADF);
+	sock = tun_get_socket(file);
+	if (IS_ERR(sock))
+		fput(file);
+	return sock;
+}
+
+static struct socket *get_socket(int fd)
+{
+	struct socket *sock;
+	if (fd == -1)
+		return NULL;
+	sock = get_raw_socket(fd);
+	if (!IS_ERR(sock))
+		return sock;
+	sock = get_tun_socket(fd);
+	if (!IS_ERR(sock))
+		return sock;
+	return ERR_PTR(-ENOTSOCK);
+}
+
+static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
+{
+	struct socket *sock, *oldsock = NULL;
+	struct vhost_virtqueue *vq;
+	int r;
+
+	mutex_lock(&n->dev.mutex);
+	r = vhost_dev_check_owner(&n->dev);
+	if (r)
+		goto done;
+
+	if (index >= VHOST_NET_VQ_MAX) {
+		r = -ENOBUFS;
+		goto done;
+	}
+	vq = n->vqs + index;
+	mutex_lock(&vq->mutex);
+	sock = get_socket(fd);
+	if (IS_ERR(sock)) {
+		r = PTR_ERR(sock);
+		goto done;
+	}
+
+	/* start polling new socket */
+	oldsock = vq->private_data;
+	if (sock == oldsock)
+		goto done;
+
+	vhost_net_disable_vq(n, index);
+	rcu_assign_pointer(vq->private_data, sock);
+	vhost_net_enable_vq(n, index);
+	mutex_unlock(&vq->mutex);
+done:
+	mutex_unlock(&n->dev.mutex);
+	if (oldsock) {
+		vhost_net_flush_vq(n, index);
+		fput(oldsock->file);
+	}
+	return r;
+}
+
+static long vhost_net_reset_owner(struct vhost_net *n)
+{
+	struct socket *tx_sock = NULL;
+	struct socket *rx_sock = NULL;
+	long r;
+	mutex_lock(&n->dev.mutex);
+	r = vhost_dev_check_owner(&n->dev);
+	if (r)
+		goto done;
+	vhost_net_stop(n, &tx_sock, &rx_sock);
+	vhost_net_flush(n);
+	r = vhost_dev_reset_owner(&n->dev);
+done:
+	mutex_unlock(&n->dev.mutex);
+	if (tx_sock)
+		fput(tx_sock->file);
+	if (rx_sock)
+		fput(rx_sock->file);
+	return r;
+}
+
+static void vhost_net_set_features(struct vhost_net *n, u64 features)
+{
+	size_t hdr_size = features & (1 << VHOST_NET_F_VIRTIO_NET_HDR) ?
+		sizeof(struct virtio_net_hdr) : 0;
+	int i;
+	mutex_unlock(&n->dev.mutex);
+	n->dev.acked_features = features;
+	smp_wmb();
+	for (i = 0; i < VHOST_NET_VQ_MAX; ++i) {
+		mutex_lock(&n->vqs[i].mutex);
+		n->vqs[i].hdr_size = hdr_size;
+		mutex_unlock(&n->vqs[i].mutex);
+	}
+	mutex_unlock(&n->dev.mutex);
+	vhost_net_flush(n);
+}
+
+static long vhost_net_ioctl(struct file *f, unsigned int ioctl,
+			    unsigned long arg)
+{
+	struct vhost_net *n = f->private_data;
+	void __user *argp = (void __user *)arg;
+	u32 __user *featurep = argp;
+	struct vhost_vring_file backend;
+	u64 features;
+	int r;
+	switch (ioctl) {
+	case VHOST_NET_SET_BACKEND:
+		r = copy_from_user(&backend, argp, sizeof backend);
+		if (r < 0)
+			return r;
+		return vhost_net_set_backend(n, backend.index, backend.fd);
+	case VHOST_GET_FEATURES:
+		features = VHOST_FEATURES;
+		return put_user(features, featurep);
+	case VHOST_ACK_FEATURES:
+		r = get_user(features, featurep);
+		/* No features for now */
+		if (r < 0)
+			return r;
+		if (features & ~VHOST_FEATURES)
+			return -EOPNOTSUPP;
+		vhost_net_set_features(n, features);
+		return 0;
+	case VHOST_RESET_OWNER:
+		return vhost_net_reset_owner(n);
+	default:
+		r = vhost_dev_ioctl(&n->dev, ioctl, arg);
+		vhost_net_flush(n);
+		return r;
+	}
+}
+
+#ifdef CONFIG_COMPAT
+static long vhost_net_compat_ioctl(struct file *f, unsigned int ioctl,
+				   unsigned long arg)
+{
+	return vhost_net_ioctl(f, ioctl, (unsigned long)compat_ptr(arg));
+}
+#endif
+
+const static struct file_operations vhost_net_fops = {
+	.owner          = THIS_MODULE,
+	.release        = vhost_net_release,
+	.unlocked_ioctl = vhost_net_ioctl,
+#ifdef CONFIG_COMPAT
+	.compat_ioctl   = vhost_net_compat_ioctl,
+#endif
+	.open           = vhost_net_open,
+};
+
+static struct miscdevice vhost_net_misc = {
+	VHOST_NET_MINOR,
+	"vhost-net",
+	&vhost_net_fops,
+};
+
+int vhost_net_init(void)
+{
+	int r = vhost_init();
+	if (r)
+		goto err_init;
+	r = misc_register(&vhost_net_misc);
+	if (r)
+		goto err_reg;
+	return 0;
+err_reg:
+	vhost_cleanup();
+err_init:
+	return r;
+
+}
+module_init(vhost_net_init);
+
+void vhost_net_exit(void)
+{
+	misc_deregister(&vhost_net_misc);
+	vhost_cleanup();
+}
+module_exit(vhost_net_exit);
+
+MODULE_VERSION("0.0.1");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Michael S. Tsirkin");
+MODULE_DESCRIPTION("Host kernel accelerator for virtio net");
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
new file mode 100644
index 0000000..9e2ecc3
--- /dev/null
+++ b/drivers/vhost/vhost.c
@@ -0,0 +1,970 @@ 
+/* Copyright (C) 2009 Red Hat, Inc.
+ * Copyright (C) 2006 Rusty Russell IBM Corporation
+ *
+ * Author: Michael S. Tsirkin <mst@redhat.com>
+ *
+ * Inspiration, some code, and most witty comments come from
+ * Documentation/lguest/lguest.c, by Rusty Russell
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.
+ *
+ * Generic code for virtio server in host kernel.
+ */
+
+#include <linux/eventfd.h>
+#include <linux/vhost.h>
+#include <linux/virtio_net.h>
+#include <linux/mm.h>
+#include <linux/miscdevice.h>
+#include <linux/mutex.h>
+#include <linux/workqueue.h>
+#include <linux/rcupdate.h>
+#include <linux/poll.h>
+#include <linux/file.h>
+#include <linux/highmem.h>
+
+#include <linux/net.h>
+#include <linux/if_packet.h>
+#include <linux/if_arp.h>
+
+#include <net/sock.h>
+
+#include "vhost.h"
+
+enum {
+	VHOST_MEMORY_MAX_NREGIONS = 64,
+	VHOST_MEMORY_F_LOG = 0x1,
+};
+
+static struct workqueue_struct *vhost_workqueue;
+
+static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
+			    poll_table *pt)
+{
+	struct vhost_poll *poll;
+	poll = container_of(pt, struct vhost_poll, table);
+
+	poll->wqh = wqh;
+	add_wait_queue(wqh, &poll->wait);
+}
+
+static int vhost_poll_wakeup(wait_queue_t *wait, unsigned mode, int sync,
+			     void *key)
+{
+	struct vhost_poll *poll;
+	poll = container_of(wait, struct vhost_poll, wait);
+	if (!((unsigned long)key & poll->mask))
+		return 0;
+
+	queue_work(vhost_workqueue, &poll->work);
+	return 0;
+}
+
+/* Init poll structure */
+void vhost_poll_init(struct vhost_poll *poll, work_func_t func,
+		     unsigned long mask)
+{
+	INIT_WORK(&poll->work, func);
+	init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup);
+	init_poll_funcptr(&poll->table, vhost_poll_func);
+	poll->mask = mask;
+}
+
+/* Start polling a file. We add ourselves to file's wait queue. The caller must
+ * keep a reference to a file until after vhost_poll_stop is called. */
+void vhost_poll_start(struct vhost_poll *poll, struct file *file)
+{
+	unsigned long mask;
+	mask = file->f_op->poll(file, &poll->table);
+	if (mask)
+		vhost_poll_wakeup(&poll->wait, 0, 0, (void *)mask);
+}
+
+/* Stop polling a file. After this function returns, it becomes safe to drop the
+ * file reference. You must also flush afterwards. */
+void vhost_poll_stop(struct vhost_poll *poll)
+{
+	remove_wait_queue(poll->wqh, &poll->wait);
+}
+
+/* Flush any work that has been scheduled. When calling this, don't hold any
+ * locks that are also used by the callback. */
+void vhost_poll_flush(struct vhost_poll *poll)
+{
+	flush_work(&poll->work);
+}
+
+void vhost_poll_queue(struct vhost_poll *poll)
+{
+	queue_work(vhost_workqueue, &poll->work);
+}
+
+long vhost_dev_init(struct vhost_dev *dev,
+		    struct vhost_virtqueue *vqs, int nvqs)
+{
+	int i;
+	dev->vqs = vqs;
+	dev->nvqs = nvqs;
+	mutex_init(&dev->mutex);
+
+	for (i = 0; i < dev->nvqs; ++i) {
+		dev->vqs[i].dev = dev;
+		mutex_init(&dev->vqs[i].mutex);
+		if (dev->vqs[i].handle_kick)
+			vhost_poll_init(&dev->vqs[i].poll,
+					dev->vqs[i].handle_kick,
+					POLLIN);
+	}
+	return 0;
+}
+
+/* Caller should have device mutex */
+long vhost_dev_check_owner(struct vhost_dev *dev)
+{
+	/* Are you the owner? If not, I don't think you mean to do that */
+	return dev->mm == current->mm ? 0 : -EPERM;
+}
+
+/* Caller should have device mutex */
+static long vhost_dev_set_owner(struct vhost_dev *dev)
+{
+	/* Is there an owner already? */
+	if (dev->mm)
+		return -EBUSY;
+	/* No owner, become one */
+	dev->mm = get_task_mm(current);
+	return 0;
+}
+
+/* Caller should have device mutex */
+long vhost_dev_reset_owner(struct vhost_dev *dev)
+{
+	struct vhost_memory *memory;
+
+	/* Restore memory to default 1:1 mapping. */
+	memory = kzalloc(offsetof(struct vhost_memory, regions) +
+			 2 * sizeof *memory->regions, GFP_KERNEL);
+	if (!memory)
+		return -ENOMEM;
+
+	vhost_dev_cleanup(dev);
+
+	memory->nregions = 2;
+	memory->regions[0].guest_phys_addr = 1;
+	memory->regions[0].userspace_addr = 1;
+	memory->regions[0].memory_size = ~0ULL;
+	memory->regions[1].guest_phys_addr = 0;
+	memory->regions[1].userspace_addr = 0;
+	memory->regions[1].memory_size = 1;
+	dev->memory = memory;
+	return 0;
+}
+
+/* Caller should have device mutex */
+void vhost_dev_cleanup(struct vhost_dev *dev)
+{
+	int i;
+	for (i = 0; i < dev->nvqs; ++i) {
+		if (dev->vqs[i].kick && dev->vqs[i].handle_kick) {
+			vhost_poll_stop(&dev->vqs[i].poll);
+			vhost_poll_flush(&dev->vqs[i].poll);
+		}
+		if (dev->vqs[i].error_ctx)
+			eventfd_ctx_put(dev->vqs[i].error_ctx);
+		if (dev->vqs[i].error)
+			fput(dev->vqs[i].error);
+		if (dev->vqs[i].kick)
+			fput(dev->vqs[i].kick);
+		if (dev->vqs[i].call_ctx)
+			eventfd_ctx_put(dev->vqs[i].call_ctx);
+		if (dev->vqs[i].call)
+			fput(dev->vqs[i].call);
+		dev->vqs[i].error_ctx = NULL;
+		dev->vqs[i].error = NULL;
+		dev->vqs[i].kick = NULL;
+		dev->vqs[i].call_ctx = NULL;
+		dev->vqs[i].call = NULL;
+	}
+	if (dev->log_ctx)
+		eventfd_ctx_put(dev->log_ctx);
+	dev->log_ctx = NULL;
+	if (dev->log_file)
+		fput(dev->log_file);
+	dev->log_file = NULL;
+	/* No one will access memory at this point */
+	kfree(dev->memory);
+	dev->memory = NULL;
+	if (dev->mm)
+		mmput(dev->mm);
+	dev->mm = NULL;
+}
+
+static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
+{
+	struct vhost_memory mem, *newmem, *oldmem;
+	unsigned long size = offsetof(struct vhost_memory, regions);
+	long r;
+	r = copy_from_user(&mem, m, size);
+	if (r)
+		return r;
+	if (mem.padding)
+		return -EOPNOTSUPP;
+	if (mem.nregions > VHOST_MEMORY_MAX_NREGIONS)
+		return -E2BIG;
+	newmem = kmalloc(size + mem.nregions * sizeof *m->regions, GFP_KERNEL);
+	if (!newmem)
+		return -ENOMEM;
+
+	memcpy(newmem, &mem, size);
+	r = copy_from_user(newmem->regions, m->regions,
+			   mem.nregions * sizeof *m->regions);
+	if (r) {
+		kfree(newmem);
+		return r;
+	}
+	oldmem = d->memory;
+	rcu_assign_pointer(d->memory, newmem);
+	synchronize_rcu();
+	kfree(oldmem);
+	return 0;
+}
+
+static int init_used(struct vhost_virtqueue *vq)
+{
+	int r = put_user(vq->used_flags, &vq->used->flags);
+	if (r)
+		return r;
+	return get_user(vq->last_used_idx, &vq->used->idx);
+}
+
+static long vhost_set_vring(struct vhost_dev *d, int ioctl, void __user *argp)
+{
+	struct file *eventfp, *filep = NULL,
+		    *pollstart = NULL, *pollstop = NULL;
+	struct eventfd_ctx *ctx = NULL;
+	u32 __user *idxp = argp;
+	struct vhost_virtqueue *vq;
+	struct vhost_vring_state s;
+	struct vhost_vring_file f;
+	struct vhost_vring_addr a;
+	u32 idx;
+	long r;
+
+	r = get_user(idx, idxp);
+	if (r < 0)
+		return r;
+	if (idx > d->nvqs)
+		return -ENOBUFS;
+
+	vq = d->vqs + idx;
+
+	mutex_lock(&vq->mutex);
+
+	switch (ioctl) {
+	case VHOST_SET_VRING_NUM:
+		r = copy_from_user(&s, argp, sizeof s);
+		if (r < 0)
+			break;
+		if (s.num > 0xffff) {
+			r = -EINVAL;
+			break;
+		}
+		vq->num = s.num;
+		break;
+	case VHOST_SET_VRING_BASE:
+		r = copy_from_user(&s, argp, sizeof s);
+		if (r < 0)
+			break;
+		if (s.num > 0xffff) {
+			r = -EINVAL;
+			break;
+		}
+		vq->avail_idx = vq->last_avail_idx = s.num;
+		break;
+	case VHOST_GET_VRING_BASE:
+		s.index = idx;
+		s.num = vq->last_avail_idx;
+		r = copy_to_user(argp, &s, sizeof s);
+		break;
+	case VHOST_SET_VRING_DESC:
+		r = copy_from_user(&a, argp, sizeof a);
+		if (r < 0)
+			break;
+		if (a.padding) {
+			r = -EOPNOTSUPP;
+			break;
+		}
+		if ((u64)(unsigned long)a.user_addr != a.user_addr) {
+			r = -EFAULT;
+			break;
+		}
+		vq->desc = (void __user *)(unsigned long)a.user_addr;
+		break;
+	case VHOST_SET_VRING_AVAIL:
+		r = copy_from_user(&a, argp, sizeof a);
+		if (r < 0)
+			break;
+		if (a.padding) {
+			r = -EOPNOTSUPP;
+			break;
+		}
+		if ((u64)(unsigned long)a.user_addr != a.user_addr) {
+			r = -EFAULT;
+			break;
+		}
+		if (a.user_addr & (sizeof *vq->avail->ring - 1)) {
+			r = -EINVAL;
+			break;
+		}
+		vq->avail = (void __user *)(unsigned long)a.user_addr;
+		/* Forget the cached index value. */
+		vq->avail_idx = vq->last_avail_idx;
+		break;
+	case VHOST_SET_VRING_USED:
+		r = copy_from_user(&a, argp, sizeof a);
+		if (r < 0)
+			break;
+		if (a.padding) {
+			r = -EOPNOTSUPP;
+			break;
+		}
+		if ((u64)(unsigned long)a.user_addr != a.user_addr) {
+			r = -EFAULT;
+			break;
+		}
+		if (a.user_addr & (sizeof *vq->used->ring - 1)) {
+			r = -EINVAL;
+			break;
+		}
+		vq->used = (void __user *)(unsigned long)a.user_addr;
+		r = init_used(vq);
+		if (r)
+			break;
+		break;
+	case VHOST_SET_VRING_LOG:
+		r = copy_from_user(&a, argp, sizeof a);
+		if (r < 0)
+			break;
+		if (a.padding) {
+			r = -EOPNOTSUPP;
+			break;
+		}
+		if (a.user_addr == VHOST_VRING_LOG_DISABLE) {
+			vq->log_used = false;
+			break;
+		}
+		if (a.user_addr & (sizeof *vq->used->ring - 1)) {
+			r = -EINVAL;
+			break;
+		}
+		vq->log_used = true;
+		vq->log_addr = a.user_addr;
+		break;
+	case VHOST_SET_VRING_KICK:
+		r = copy_from_user(&f, argp, sizeof f);
+		if (r < 0)
+			break;
+		eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
+		if (IS_ERR(eventfp))
+			return PTR_ERR(eventfp);
+		if (eventfp != vq->kick) {
+			pollstop = filep = vq->kick;
+			pollstart = vq->kick = eventfp;
+		} else
+			filep = eventfp;
+		break;
+	case VHOST_SET_VRING_CALL:
+		r = copy_from_user(&f, argp, sizeof f);
+		if (r < 0)
+			break;
+		eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
+		if (IS_ERR(eventfp))
+			return PTR_ERR(eventfp);
+		if (eventfp != vq->call) {
+			filep = vq->call;
+			ctx = vq->call_ctx;
+			vq->call = eventfp;
+			vq->call_ctx = eventfp ?
+				eventfd_ctx_fileget(eventfp) : NULL;
+		} else
+			filep = eventfp;
+		break;
+	case VHOST_SET_VRING_ERR:
+		r = copy_from_user(&f, argp, sizeof f);
+		if (r < 0)
+			break;
+		eventfp = f.fd == -1 ? NULL : eventfd_fget(f.fd);
+		if (IS_ERR(eventfp))
+			return PTR_ERR(eventfp);
+		if (eventfp != vq->error) {
+			filep = vq->error;
+			vq->error = eventfp;
+			ctx = vq->error_ctx;
+			vq->error_ctx = eventfp ?
+				eventfd_ctx_fileget(eventfp) : NULL;
+		} else
+			filep = eventfp;
+		break;
+	default:
+		r = -ENOIOCTLCMD;
+	}
+
+	if (pollstop && vq->handle_kick)
+		vhost_poll_stop(&vq->poll);
+
+	if (ctx)
+		eventfd_ctx_put(ctx);
+	if (filep)
+		fput(filep);
+
+	if (pollstart && vq->handle_kick)
+		vhost_poll_start(&vq->poll, vq->kick);
+
+	mutex_unlock(&vq->mutex);
+
+	if (pollstop && vq->handle_kick)
+		vhost_poll_flush(&vq->poll);
+	return r;
+}
+
+long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, unsigned long arg)
+{
+	void __user *argp = (void __user *)arg;
+	struct file *eventfp, *filep = NULL;
+	struct eventfd_ctx *ctx = NULL;
+	u64 p;
+	long r;
+	int i, fd;
+
+	mutex_lock(&d->mutex);
+	/* If you are not the owner, you can become one */
+	if (ioctl == VHOST_SET_OWNER) {
+		r = vhost_dev_set_owner(d);
+		goto done;
+	}
+
+	/* You must be the owner to do anything else */
+	r = vhost_dev_check_owner(d);
+	if (r)
+		goto done;
+
+	switch (ioctl) {
+	case VHOST_SET_MEM_TABLE:
+		r = vhost_set_memory(d, argp);
+		break;
+	case VHOST_SET_LOG_BASE:
+		r = get_user(p, (u64 __user *)argp);
+		if (r < 0)
+			break;
+		if ((u64)(unsigned long)p != p) {
+			r = -EFAULT;
+			break;
+		}
+		for (i = 0; i < d->nvqs; ++i) {
+			mutex_lock(&d->vqs[i].mutex);
+			d->vqs[i].log_base = (void __user *)(unsigned long)p;
+			mutex_unlock(&d->vqs[i].mutex);
+		}
+		break;
+	case VHOST_SET_LOG_FD:
+		r = get_user(fd, (int __user *)argp);
+		if (r < 0)
+			break;
+		eventfp = fd == -1 ? NULL : eventfd_fget(fd);
+		if (IS_ERR(eventfp)) {
+			r = PTR_ERR(eventfp);
+			break;
+		}
+		if (eventfp != d->log_file) {
+			filep = d->log_file;
+			ctx = d->log_ctx;
+			d->log_ctx = eventfp ?
+				eventfd_ctx_fileget(eventfp) : NULL;
+		} else
+			filep = eventfp;
+		for (i = 0; i < d->nvqs; ++i) {
+			mutex_lock(&d->vqs[i].mutex);
+			d->vqs[i].log_ctx = d->log_ctx;
+			mutex_unlock(&d->vqs[i].mutex);
+		}
+		if (ctx)
+			eventfd_ctx_put(ctx);
+		if (filep)
+			fput(filep);
+		break;
+	default:
+		r = vhost_set_vring(d, ioctl, argp);
+		break;
+	}
+done:
+	mutex_unlock(&d->mutex);
+	return r;
+}
+
+static const struct vhost_memory_region *find_region(struct vhost_memory *mem,
+						     __u64 addr, __u32 len)
+{
+	struct vhost_memory_region *reg;
+	int i;
+	/* linear search is not brilliant, but we really have on the order of 6
+	 * regions in practice */
+	for (i = 0; i < mem->nregions; ++i) {
+		reg = mem->regions + i;
+		if (reg->guest_phys_addr <= addr &&
+		    reg->guest_phys_addr + reg->memory_size - 1 >= addr)
+			return reg;
+	}
+	return NULL;
+}
+
+/* TODO: This is really inefficient.  We need something like get_user()
+ * (instruction directly accesses the data, with an exception table entry
+ * returning -EFAULT). See Documentation/x86/exception-tables.txt.
+ */
+static int set_bit_to_user(int nr, void __user *addr)
+{
+	unsigned long log = (unsigned long)addr;
+	struct page *page;
+	void *base;
+	int bit = nr + (log % PAGE_SIZE) * 8;
+	int r;
+	r = get_user_pages_fast(log, 1, 1, &page);
+	if (r)
+		return r;
+	base = kmap_atomic(page, KM_USER0);
+	set_bit(bit, base);
+	kunmap_atomic(base, KM_USER0);
+	set_page_dirty_lock(page);
+	put_page(page);
+	return 0;
+}
+
+static int log_write(void __user *log_base,
+		     u64 write_address, u64 write_length)
+{
+	int r;
+	if (!write_length)
+		return 0;
+	write_address /= VHOST_PAGE_SIZE;
+	for (;;) {
+		u64 base = (u64)log_base;
+		u64 log = base + write_address / 8;
+		int bit = write_address % 8;
+		if ((u64)(unsigned long)log != log)
+			return -EFAULT;
+		r = set_bit_to_user(bit, (void __user *)log);
+		if (r < 0)
+			return r;
+		if (write_length <= VHOST_PAGE_SIZE)
+			break;
+		write_length -= VHOST_PAGE_SIZE;
+		write_address += VHOST_PAGE_SIZE;
+	}
+	return r;
+}
+
+int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
+		    unsigned int log_num, u64 len)
+{
+	int i, r;
+
+	/* Make sure data written is seen before log. */
+	wmb();
+	for (i = 0; i < log_num; ++i) {
+		u64 l = min(log[i].len, len);
+		r = log_write(vq->log_base, log[i].addr, l);
+		if (r < 0)
+			return r;
+		len -= l;
+		if (!len)
+			return 0;
+	}
+	if (vq->log_ctx)
+		eventfd_signal(vq->log_ctx, 1);
+	/* Length written exceeds what we have stored. This is a bug. */
+	BUG();
+	return 0;
+}
+
+int translate_desc(struct vhost_dev *dev, u64 addr, u32 len,
+		   struct iovec iov[], int iov_size)
+{
+	const struct vhost_memory_region *reg;
+	struct vhost_memory *mem;
+	struct iovec *_iov;
+	u64 s = 0;
+	int ret = 0;
+
+	rcu_read_lock();
+
+	mem = rcu_dereference(dev->memory);
+	while ((u64)len > s) {
+		u64 size;
+		if (ret >= iov_size) {
+			ret = -ENOBUFS;
+			break;
+		}
+		reg = find_region(mem, addr, len);
+		if (!reg) {
+			ret = -EFAULT;
+			break;
+		}
+		_iov = iov + ret;
+		size = reg->memory_size - addr + reg->guest_phys_addr;
+		_iov->iov_len = min((u64)len, size);
+		_iov->iov_base = (void *)
+			(reg->userspace_addr + addr - reg->guest_phys_addr);
+		s += size;
+		addr += size;
+		++ret;
+	}
+
+	rcu_read_unlock();
+	return ret;
+}
+
+/* Each buffer in the virtqueues is actually a chain of descriptors.  This
+ * function returns the next descriptor in the chain,
+ * or -1 if we're at the end. */
+static unsigned next_desc(struct vring_desc *desc)
+{
+	unsigned int next;
+
+	/* If this descriptor says it doesn't chain, we're done. */
+	if (!(desc->flags & VRING_DESC_F_NEXT))
+		return -1;
+
+	/* Check they're not leading us off end of descriptors. */
+	next = desc->next;
+	/* Make sure compiler knows to grab that: we don't want it changing! */
+	/* We will use the result as an index in an array, so most
+	 * architectures only need a compiler barrier here. */
+	read_barrier_depends();
+
+	return next;
+}
+
+static unsigned get_indirect(struct vhost_dev *dev, struct vhost_virtqueue *vq,
+			     struct iovec iov[],
+			     unsigned int *out_num, unsigned int *in_num,
+			     struct vhost_log *log, unsigned int *log_num,
+			     struct vring_desc *indirect)
+{
+	struct vring_desc desc;
+	unsigned int i = 0, count, found = 0;
+	int ret;
+
+	/* Sanity check */
+	if (indirect->len % sizeof desc) {
+		vq_err(vq, "Invalid length in indirect descriptor: "
+		       "len 0x%llx not multiple of 0x%zx\n",
+		       (unsigned long long)indirect->len,
+		       sizeof desc);
+		return -EINVAL;
+	}
+
+	ret = translate_desc(dev, indirect->addr, indirect->len, vq->indirect,
+			     ARRAY_SIZE(vq->indirect));
+	if (ret < 0) {
+		vq_err(vq, "Translation failure %d in indirect.\n", ret);
+		return ret;
+	}
+
+	/* We will use the result as an address to read from, so most
+	 * architectures only need a compiler barrier here. */
+	read_barrier_depends();
+
+	count = indirect->len / sizeof desc;
+	/* Buffers are chained via a 16 bit next field, so
+	 * we can have at most 2^16 of these. */
+	if (count > USHORT_MAX + 1) {
+		vq_err(vq, "Indirect buffer length too big: %d\n",
+		       indirect->len);
+		return -E2BIG;
+	}
+
+	do {
+		unsigned iov_count = *in_num + *out_num;
+		if (++found > count) {
+			vq_err(vq, "Loop detected: last one at %u "
+			       "indirect size %u\n",
+			       i, count);
+			return -EINVAL;
+		}
+		if (memcpy_fromiovec((unsigned char *)&desc, vq->indirect,
+				     sizeof desc)) {
+			vq_err(vq, "Failed indirect descriptor: idx %d, %zx\n",
+			       i, (size_t)indirect->addr + i * sizeof desc);
+			return -EINVAL;
+		}
+		if (desc.flags & VRING_DESC_F_INDIRECT) {
+			vq_err(vq, "Nested indirect descriptor: idx %d, %zx\n",
+			       i, (size_t)indirect->addr + i * sizeof desc);
+			return -EINVAL;
+		}
+
+		ret = translate_desc(dev, desc.addr, desc.len, iov + iov_count,
+				     VHOST_NET_MAX_SG - iov_count);
+		if (ret < 0) {
+			vq_err(vq, "Translation failure %d indirect idx %d\n",
+			       ret, i);
+			return ret;
+		}
+		/* If this is an input descriptor, increment that count. */
+		if (desc.flags & VRING_DESC_F_WRITE) {
+			*in_num += ret;
+			if (unlikely(log)) {
+				log[*log_num].addr = desc.addr;
+				log[*log_num].len = desc.len;
+				++*log_num;
+			}
+		} else {
+			/* If it's an output descriptor, they're all supposed
+			 * to come before any input descriptors. */
+			if (*in_num) {
+				vq_err(vq, "Indirect descriptor "
+				       "has out after in: idx %d\n", i);
+				return -EINVAL;
+			}
+			*out_num += ret;
+		}
+	} while ((i = next_desc(&desc)) != -1);
+	return 0;
+}
+
+/* This looks in the virtqueue and for the first available buffer, and converts
+ * it to an iovec for convenient access.  Since descriptors consist of some
+ * number of output then some number of input descriptors, it's actually two
+ * iovecs, but we pack them into one and note how many of each there were.
+ *
+ * This function returns the descriptor number found, or vq->num (which
+ * is never a valid descriptor number) if none was found. */
+unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
+			   struct iovec iov[],
+			   unsigned int *out_num, unsigned int *in_num,
+			   struct vhost_log *log, unsigned int *log_num)
+{
+	struct vring_desc desc;
+	unsigned int i, head, found = 0;
+	u16 last_avail_idx;
+	int ret;
+
+	/* Check it isn't doing very strange things with descriptor numbers. */
+	last_avail_idx = vq->last_avail_idx;
+	if (get_user(vq->avail_idx, &vq->avail->idx)) {
+		vq_err(vq, "Failed to access avail idx at %p\n",
+		       &vq->avail->idx);
+		return vq->num;
+	}
+
+	if ((u16)(vq->avail_idx - last_avail_idx) > vq->num) {
+		vq_err(vq, "Guest moved used index from %u to %u",
+		       last_avail_idx, vq->avail_idx);
+		return vq->num;
+	}
+
+	/* If there's nothing new since last we looked, return invalid. */
+	if (vq->avail_idx == last_avail_idx)
+		return vq->num;
+
+	/* Only get avail ring entries after they have been exposed by guest. */
+	rmb();
+
+	/* Grab the next descriptor number they're advertising, and increment
+	 * the index we've seen. */
+	if (get_user(head, &vq->avail->ring[last_avail_idx % vq->num])) {
+		vq_err(vq, "Failed to read head: idx %d address %p\n",
+		       last_avail_idx,
+		       &vq->avail->ring[last_avail_idx % vq->num]);
+		return vq->num;
+	}
+
+	/* If their number is silly, that's an error. */
+	if (head >= vq->num) {
+		vq_err(vq, "Guest says index %u > %u is available",
+		       head, vq->num);
+		return vq->num;
+	}
+
+	/* When we start there are none of either input nor output. */
+	*out_num = *in_num = 0;
+	if (unlikely(log))
+		*log_num = 0;
+
+	i = head;
+	do {
+		unsigned iov_count = *in_num + *out_num;
+		if (i >= vq->num) {
+			vq_err(vq, "Desc index is %u > %u, head = %u",
+			       i, vq->num, head);
+			return vq->num;
+		}
+		if (++found > vq->num) {
+			vq_err(vq, "Loop detected: last one at %u "
+			       "vq size %u head %u\n",
+			       i, vq->num, head);
+			return vq->num;
+		}
+		ret = copy_from_user(&desc, vq->desc + i, sizeof desc);
+		if (ret) {
+			vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
+			       i, vq->desc + i);
+			return vq->num;
+		}
+		if (desc.flags & VRING_DESC_F_INDIRECT) {
+			ret = get_indirect(dev, vq, iov, out_num, in_num,
+					   log, log_num, &desc);
+			if (ret < 0) {
+				vq_err(vq, "Failure detected "
+				       "in indirect descriptor at idx %d\n", i);
+				return vq->num;
+			}
+			continue;
+		}
+
+		ret = translate_desc(dev, desc.addr, desc.len, iov + iov_count,
+				     VHOST_NET_MAX_SG - iov_count);
+		if (ret < 0) {
+			vq_err(vq, "Translation failure %d descriptor idx %d\n",
+			       ret, i);
+			return vq->num;
+		}
+		if (desc.flags & VRING_DESC_F_WRITE) {
+			/* If this is an input descriptor,
+			 * increment that count. */
+			*in_num += ret;
+			if (unlikely(log)) {
+				log[*log_num].addr = desc.addr;
+				log[*log_num].len = desc.len;
+				++*log_num;
+			}
+		} else {
+			/* If it's an output descriptor, they're all supposed
+			 * to come before any input descriptors. */
+			if (*in_num) {
+				vq_err(vq, "Descriptor has out after in: "
+				       "idx %d\n", i);
+				return vq->num;
+			}
+			*out_num += ret;
+		}
+	} while ((i = next_desc(&desc)) != -1);
+
+	/* On success, increment avail index. */
+	vq->last_avail_idx++;
+	return head;
+}
+
+/* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
+void vhost_discard_vq_desc(struct vhost_virtqueue *vq)
+{
+	vq->last_avail_idx--;
+}
+
+/* After we've used one of their buffers, we tell them about it.  We'll then
+ * want to send them an interrupt, using vq->call. */
+int vhost_add_used(struct vhost_virtqueue *vq,
+			  unsigned int head, int len)
+{
+	struct vring_used_elem *used;
+
+	/* The virtqueue contains a ring of used buffers.  Get a pointer to the
+	 * next entry in that used ring. */
+	used = &vq->used->ring[vq->last_used_idx % vq->num];
+	if (put_user(head, &used->id)) {
+		vq_err(vq, "Failed to write used id");
+		return -EFAULT;
+	}
+	if (put_user(len, &used->len)) {
+		vq_err(vq, "Failed to write used len");
+		return -EFAULT;
+	}
+	/* Make sure buffer is written before we update index. */
+	wmb();
+	if (put_user(vq->last_used_idx + 1, &vq->used->idx)) {
+		vq_err(vq, "Failed to increment used idx");
+		return -EFAULT;
+	}
+	if (unlikely(vq->log_used)) {
+		/* Make sure data is seen before log. */
+		wmb();
+		log_write(vq->log_base, vq->log_addr + sizeof *vq->used->ring *
+			  (vq->last_used_idx % vq->num),
+			  sizeof *vq->used->ring);
+		log_write(vq->log_base, vq->log_addr, sizeof *vq->used->ring);
+		if (vq->log_ctx)
+			eventfd_signal(vq->log_ctx, 1);
+	}
+	vq->last_used_idx++;
+	return 0;
+}
+
+/* This actually sends the interrupt for this virtqueue */
+void vhost_trigger_irq(struct vhost_dev *dev, struct vhost_virtqueue *vq)
+{
+	__u16 flags = 0;
+	if (get_user(flags, &vq->avail->flags)) {
+		vq_err(vq, "Failed to get flags");
+		return;
+	}
+
+	/* If they don't want an interrupt, don't send one, unless empty. */
+	if ((flags & VRING_AVAIL_F_NO_INTERRUPT) &&
+	    (vq->avail_idx != vq->last_avail_idx ||
+	     !vhost_has_feature(dev, VIRTIO_F_NOTIFY_ON_EMPTY)))
+		return;
+
+	/* Send the Guest an interrupt tell them we used something up. */
+	if (vq->call_ctx)
+		eventfd_signal(vq->call_ctx, 1);
+}
+
+/* And here's the combo meal deal.  Supersize me! */
+void vhost_add_used_and_trigger(struct vhost_dev *dev,
+				struct vhost_virtqueue *vq,
+				unsigned int head, int len)
+{
+	vhost_add_used(vq, head, len);
+	vhost_trigger_irq(dev, vq);
+}
+
+/* OK, now we need to know about added descriptors. */
+bool vhost_notify(struct vhost_virtqueue *vq)
+{
+	int r;
+	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
+		return false;
+	vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;
+	r = put_user(vq->used_flags, &vq->used->flags);
+	if (r)
+		vq_err(vq, "Failed to disable notification: %d\n", r);
+	/* They could have slipped one in as we were doing that: make
+	 * sure it's written, tell caller it needs to check again. */
+	mb();
+	return true;
+}
+
+/* We don't need to be notified again. */
+void vhost_no_notify(struct vhost_virtqueue *vq)
+{
+	int r;
+	if (vq->used_flags & VRING_USED_F_NO_NOTIFY)
+		return;
+	vq->used_flags |= VRING_USED_F_NO_NOTIFY;
+	r = put_user(vq->used_flags, &vq->used->flags);
+	if (r)
+		vq_err(vq, "Failed to enable notification: %d\n", r);
+}
+
+int vhost_init(void)
+{
+	vhost_workqueue = create_singlethread_workqueue("vhost");
+	if (!vhost_workqueue)
+		return -ENOMEM;
+	return 0;
+}
+
+void vhost_cleanup(void)
+{
+	destroy_workqueue(vhost_workqueue);
+}
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
new file mode 100644
index 0000000..d3900de
--- /dev/null
+++ b/drivers/vhost/vhost.h
@@ -0,0 +1,158 @@ 
+#ifndef _VHOST_H
+#define _VHOST_H
+
+#include <linux/eventfd.h>
+#include <linux/vhost.h>
+#include <linux/mm.h>
+#include <linux/mutex.h>
+#include <linux/workqueue.h>
+#include <linux/poll.h>
+#include <linux/file.h>
+#include <linux/skbuff.h>
+#include <linux/uio.h>
+#include <linux/virtio_config.h>
+#include <linux/virtio_ring.h>
+
+struct vhost_device;
+
+enum {
+	VHOST_NET_MAX_SG = MAX_SKB_FRAGS + 2,
+};
+
+/* Poll a file (eventfd or socket) */
+/* Note: there's nothing vhost specific about this structure. */
+struct vhost_poll {
+	poll_table                table;
+	wait_queue_head_t        *wqh;
+	wait_queue_t              wait;
+	/* struct which will handle all actual work. */
+	struct work_struct        work;
+	unsigned long		  mask;
+};
+
+void vhost_poll_init(struct vhost_poll *poll, work_func_t func,
+		     unsigned long mask);
+void vhost_poll_start(struct vhost_poll *poll, struct file *file);
+void vhost_poll_stop(struct vhost_poll *poll);
+void vhost_poll_flush(struct vhost_poll *poll);
+void vhost_poll_queue(struct vhost_poll *poll);
+
+struct vhost_log {
+	u64 addr;
+	u64 len;
+};
+
+/* The virtqueue structure describes a queue attached to a device. */
+struct vhost_virtqueue {
+	struct vhost_dev *dev;
+
+	/* The actual ring of buffers. */
+	struct mutex mutex;
+	unsigned int num;
+	struct vring_desc __user *desc;
+	struct vring_avail __user *avail;
+	struct vring_used __user *used;
+	struct file *kick;
+	struct file *call;
+	struct file *error;
+	struct eventfd_ctx *call_ctx;
+	struct eventfd_ctx *error_ctx;
+	struct eventfd_ctx *log_ctx;
+
+	struct vhost_poll poll;
+
+	/* The routine to call when the Guest pings us, or timeout. */
+	work_func_t handle_kick;
+
+	/* Last available index we saw. */
+	u16 last_avail_idx;
+
+	/* Caches available index value from user. */
+	u16 avail_idx;
+
+	/* Last index we used. */
+	u16 last_used_idx;
+
+	/* Used flags */
+	u16 used_flags;
+
+	/* Log writes to used structure. */
+	bool log_used;
+	u64 log_addr;
+
+	struct iovec indirect[VHOST_NET_MAX_SG];
+	struct iovec iov[VHOST_NET_MAX_SG];
+	struct iovec hdr[VHOST_NET_MAX_SG];
+	size_t hdr_size;
+	/* We use a kind of RCU to access private pointer.
+	 * All readers access it from workqueue, which makes it possible to
+	 * flush the workqueue instead of synchronize_rcu. Therefore readers do
+	 * not need to call rcu_read_lock/rcu_read_unlock: the beginning of
+	 * work item execution acts instead of rcu_read_lock() and the end of
+	 * work item execution acts instead of rcu_read_lock().
+	 * Writers use virtqueue mutex. */
+	void *private_data;
+	/* Log write descriptors */
+	void __user *log_base;
+	struct vhost_log log[VHOST_NET_MAX_SG];
+};
+
+struct vhost_dev {
+	/* Readers use RCU to access memory table pointer
+	 * log base pointer and features.
+	 * Writers use mutex below.*/
+	struct vhost_memory *memory;
+	struct mm_struct *mm;
+	struct mutex mutex;
+	unsigned acked_features;
+	struct vhost_virtqueue *vqs;
+	int nvqs;
+	struct file *log_file;
+	struct eventfd_ctx *log_ctx;
+};
+
+long vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue *vqs, int nvqs);
+long vhost_dev_check_owner(struct vhost_dev *);
+long vhost_dev_reset_owner(struct vhost_dev *);
+void vhost_dev_cleanup(struct vhost_dev *);
+long vhost_dev_ioctl(struct vhost_dev *, unsigned int ioctl, unsigned long arg);
+
+unsigned vhost_get_vq_desc(struct vhost_dev *, struct vhost_virtqueue *,
+			   struct iovec iov[],
+			   unsigned int *out_num, unsigned int *in_num,
+			   struct vhost_log *log, unsigned int *log_num);
+void vhost_discard_vq_desc(struct vhost_virtqueue *);
+
+int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
+void vhost_trigger_irq(struct vhost_dev *, struct vhost_virtqueue *);
+void vhost_add_used_and_trigger(struct vhost_dev *, struct vhost_virtqueue *,
+				unsigned int head, int len);
+void vhost_no_notify(struct vhost_virtqueue *);
+bool vhost_notify(struct vhost_virtqueue *);
+
+int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
+		    unsigned int log_num, u64 len);
+
+int vhost_init(void);
+void vhost_cleanup(void);
+
+#define vq_err(vq, fmt, ...) do {                                  \
+		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
+		if ((vq)->error_ctx)                               \
+				eventfd_signal((vq)->error_ctx, 1);\
+	} while (0)
+
+enum {
+	VHOST_FEATURES = (1 << VIRTIO_F_NOTIFY_ON_EMPTY) |
+			 (1 << VIRTIO_RING_F_INDIRECT_DESC) |
+			 (1 << VHOST_F_LOG_ALL) |
+			 (1 << VHOST_NET_F_VIRTIO_NET_HDR),
+};
+
+static inline int vhost_has_feature(struct vhost_dev *dev, int bit)
+{
+	unsigned acked_features = rcu_dereference(dev->acked_features);
+	return acked_features & (1 << bit);
+}
+
+#endif
diff --git a/include/linux/Kbuild b/include/linux/Kbuild
index 3f384d4..b6335c6 100644
--- a/include/linux/Kbuild
+++ b/include/linux/Kbuild
@@ -361,6 +361,7 @@  unifdef-y += uio.h
 unifdef-y += unistd.h
 unifdef-y += usbdevice_fs.h
 unifdef-y += utsname.h
+unifdef-y += vhost.h
 unifdef-y += videodev2.h
 unifdef-y += videodev.h
 unifdef-y += virtio_config.h
diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h
index adaf3c1..8b5f7cc 100644
--- a/include/linux/miscdevice.h
+++ b/include/linux/miscdevice.h
@@ -30,6 +30,7 @@ 
 #define HPET_MINOR		228
 #define FUSE_MINOR		229
 #define KVM_MINOR		232
+#define VHOST_NET_MINOR		233
 #define MISC_DYNAMIC_MINOR	255
 
 struct device;
diff --git a/include/linux/vhost.h b/include/linux/vhost.h
new file mode 100644
index 0000000..f66142f
--- /dev/null
+++ b/include/linux/vhost.h
@@ -0,0 +1,126 @@ 
+#ifndef _LINUX_VHOST_H
+#define _LINUX_VHOST_H
+/* Userspace interface for in-kernel virtio accelerators. */
+
+/* vhost is used to reduce the number of system calls involved in virtio.
+ *
+ * Existing virtio net code is used in the guest without modification.
+ *
+ * This header includes interface used by userspace hypervisor for
+ * device configuration.
+ */
+
+#include <linux/types.h>
+#include <linux/compiler.h>
+#include <linux/ioctl.h>
+#include <linux/virtio_config.h>
+#include <linux/virtio_ring.h>
+
+struct vhost_vring_state {
+	unsigned int index;
+	unsigned int num;
+};
+
+struct vhost_vring_file {
+	unsigned int index;
+	int fd; /* Pass -1 to unbind from file. */
+
+};
+
+struct vhost_vring_addr {
+	unsigned int index;
+	unsigned int padding;
+	__u64 user_addr;
+};
+
+struct vhost_memory_region {
+	__u64 guest_phys_addr;
+	__u64 memory_size; /* bytes */
+	__u64 userspace_addr;
+	__u64 flags_padding; /* No flags are currently specified. */
+};
+
+/* All region addresses and sizes must be 4K aligned. */
+#define VHOST_PAGE_SIZE 0x1000
+
+struct vhost_memory {
+	__u32 nregions;
+	__u32 padding;
+	struct vhost_memory_region regions[0];
+};
+
+/* ioctls */
+
+#define VHOST_VIRTIO 0xAF
+
+/* Features bitmask for forward compatibility.  Transport bits are used for
+ * vhost specific features. */
+#define VHOST_GET_FEATURES	_IOR(VHOST_VIRTIO, 0x00, __u64)
+#define VHOST_ACK_FEATURES	_IOW(VHOST_VIRTIO, 0x00, __u64)
+
+/* Set current process as the (exclusive) owner of this file descriptor.  This
+ * must be called before any other vhost command.  Further calls to
+ * VHOST_OWNER_SET fail until VHOST_OWNER_RESET is called. */
+#define VHOST_SET_OWNER _IO(VHOST_VIRTIO, 0x01)
+/* Give up ownership, and reset the device to default values.
+ * Allows subsequent call to VHOST_OWNER_SET to succeed. */
+#define VHOST_RESET_OWNER _IO(VHOST_VIRTIO, 0x02)
+
+/* Set up/modify memory layout */
+#define VHOST_SET_MEM_TABLE	_IOW(VHOST_VIRTIO, 0x03, struct vhost_memory)
+
+/* Write logging setup. */
+/* Memory writes can optionally be logged by setting bit at an offset
+ * (calculated from the physical address) from specified log base.
+ * The bit is set using an atomic 32 bit operation. */
+/* Set base address for logging. */
+#define VHOST_SET_LOG_BASE _IOW(VHOST_VIRTIO, 0x04, __u64)
+/* Specify an eventfd file descriptor to signal on log write. */
+#define VHOST_SET_LOG_FD _IOW(VHOST_VIRTIO, 0x07, int)
+
+/* Ring setup. These parameters can not be modified while ring is running
+ * (bound to a device). */
+/* Set number of descriptors in ring */
+#define VHOST_SET_VRING_NUM _IOW(VHOST_VIRTIO, 0x10, struct vhost_vring_state)
+/* Start of array of descriptors (virtually contiguous) */
+#define VHOST_SET_VRING_DESC _IOW(VHOST_VIRTIO, 0x11, struct vhost_vring_addr)
+/* Used structure address. Must be 32 bit aligned */
+#define VHOST_SET_VRING_USED _IOW(VHOST_VIRTIO, 0x12, struct vhost_vring_addr)
+/* Available structure address. Must be 16 bit aligned */
+#define VHOST_SET_VRING_AVAIL _IOW(VHOST_VIRTIO, 0x13, struct vhost_vring_addr)
+/* Base value where queue looks for available descriptors */
+#define VHOST_SET_VRING_BASE _IOW(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
+/* Get accessor: reads index, writes value in num */
+#define VHOST_GET_VRING_BASE _IOWR(VHOST_VIRTIO, 0x14, struct vhost_vring_state)
+
+/* Logging support. Can be modified while ring is running. */
+/* Log writes to used structure, at offset calculated from specified address.
+ * Address must be 32 bit aligned. Pass 0x1 to disable logging. */
+#define VHOST_SET_VRING_LOG _IOW(VHOST_VIRTIO, 0x18, struct vhost_vring_addr)
+#define VHOST_VRING_LOG_DISABLE (0x1)
+
+/* The following ioctls use eventfd file descriptors to signal and poll
+ * for events. */
+
+/* Set eventfd to poll for added buffers */
+#define VHOST_SET_VRING_KICK _IOW(VHOST_VIRTIO, 0x20, struct vhost_vring_file)
+/* Set eventfd to signal when buffers have beed used */
+#define VHOST_SET_VRING_CALL _IOW(VHOST_VIRTIO, 0x21, struct vhost_vring_file)
+/* Set eventfd to signal an error */
+#define VHOST_SET_VRING_ERR _IOW(VHOST_VIRTIO, 0x22, struct vhost_vring_file)
+
+/* VHOST_NET specific defines */
+
+/* Attach virtio net ring to a raw socket, or tap device.
+ * The socket must be already bound to an ethernet device, this device will be
+ * used for transmit.  Pass fd -1 to unbind from the socket and the transmit
+ * device.  This can be used to stop the ring (e.g. for migration). */
+#define VHOST_NET_SET_BACKEND _IOW(VHOST_VIRTIO, 0x30, struct vhost_vring_file)
+
+/* Feature bits */
+/* Log all write descriptors. Can be changed while device is active. */
+#define VHOST_F_LOG_ALL 26
+/* vhost-net should add virtio_net_hdr for RX, and strip for TX packets. */
+#define VHOST_NET_F_VIRTIO_NET_HDR 27
+
+#endif