[2/2] vhost_net: a kernel-level virtio server

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)
- 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 backend 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
tun-like device. In this version I only support raw socket as a backend,
which can be bound to e.g. SR IOV, or to macvlan device.  Backend is
also configured by userspace, including vlan/mac etc.

Status:
This works for me, and I haven't see any crashes.
I have not run any benchmarks yet, compared to userspace, I expect to
see improved latency (as I save up to 4 system calls per packet) but not
yet bandwidth/CPU (as TSO and interrupt mitigation are not yet supported).

Features that I plan to look at in the future:
- TSO
- interrupt mitigation
- zero copy

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

---
 MAINTAINERS                |   10 +
 arch/x86/kvm/Kconfig       |    1 +
 drivers/Makefile           |    1 +
 drivers/block/virtio_blk.c |    3 +
 drivers/vhost/Kconfig      |   11 +
 drivers/vhost/Makefile     |    2 +
 drivers/vhost/net.c        |  462 ++++++++++++++++++++++++++++++
 drivers/vhost/vhost.c      |  663 ++++++++++++++++++++++++++++++++++++++++++++
 drivers/vhost/vhost.h      |  108 +++++++
 include/linux/Kbuild       |    1 +
 include/linux/miscdevice.h |    1 +
 include/linux/vhost.h      |  100 +++++++
 12 files changed, 1363 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

On Monday 10 August 2009, Michael S. Tsirkin wrote:
> 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.

Very nice, I loved reading it. It's getting rather late in my time
zone, so this comments only on the network driver. I'll go through
the rest tomorrow.

> @@ -293,6 +293,7 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
>  		err = PTR_ERR(vblk->vq);
>  		goto out_free_vblk;
>  	}
> +	printk(KERN_ERR "vblk->vq = %p\n", vblk->vq);
>  
>  	vblk->pool = mempool_create_kmalloc_pool(1,sizeof(struct virtblk_req));
>  	if (!vblk->pool) {
> @@ -383,6 +384,8 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
>  	if (!err)
>  		blk_queue_logical_block_size(vblk->disk->queue, blk_size);
>  
> +	printk(KERN_ERR "virtio_config_val returned %d\n", err);
> +
>  	add_disk(vblk->disk);
>  	return 0;

I guess you meant to remove these before submitting.

> +static void handle_tx_kick(struct work_struct *work);
> +static void handle_rx_kick(struct work_struct *work);
> +static void handle_tx_net(struct work_struct *work);
> +static void handle_rx_net(struct work_struct *work);

[style] I think the code gets more readable if you reorder it
so that you don't need forward declarations for static functions.

> +static long vhost_net_reset_owner(struct vhost_net *n)
> +{
> +	struct socket *sock = NULL;
> +	long r;
> +	mutex_lock(&n->dev.mutex);
> +	r = vhost_dev_check_owner(&n->dev);
> +	if (r)
> +		goto done;
> +	sock = vhost_net_stop(n);
> +	r = vhost_dev_reset_owner(&n->dev);
> +done:
> +	mutex_unlock(&n->dev.mutex);
> +	if (sock)
> +		fput(sock->file);
> +	return r;
> +}

what is the difference between vhost_net_reset_owner(n)
and vhost_net_set_socket(n, -1)?

> +
> +static struct file_operations vhost_net_fops = {
> +	.owner          = THIS_MODULE,
> +	.release        = vhost_net_release,
> +	.unlocked_ioctl = vhost_net_ioctl,
> +	.open           = vhost_net_open,
> +};

This is missing a compat_ioctl pointer. It should simply be

static long vhost_net_compat_ioctl(struct file *f,
			unsigned int ioctl, unsigned long arg)
{
	return f, ioctl, (unsigned long)compat_ptr(arg);
}

> +/* Bits from fs/aio.c. TODO: export and use from there? */
> +/*
> + * use_mm
> + *	Makes the calling kernel thread take on the specified
> + *	mm context.
> + *	Called by the retry thread execute retries within the
> + *	iocb issuer's mm context, so that copy_from/to_user
> + *	operations work seamlessly for aio.
> + *	(Note: this routine is intended to be called only
> + *	from a kernel thread context)
> + */
> +static void use_mm(struct mm_struct *mm)
> +{
> +	struct mm_struct *active_mm;
> +	struct task_struct *tsk = current;
> +
> +	task_lock(tsk);
> +	active_mm = tsk->active_mm;
> +	atomic_inc(&mm->mm_count);
> +	tsk->mm = mm;
> +	tsk->active_mm = mm;
> +	switch_mm(active_mm, mm, tsk);
> +	task_unlock(tsk);
> +
> +	mmdrop(active_mm);
> +}

Why do you need a kernel thread here? If the data transfer functions
all get called from a guest intercept, shouldn't you already be
in the right mm?

> +static void handle_tx(struct vhost_net *net)
> +{
> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> +	unsigned head, out, in;
> +	struct msghdr msg = {
> +		.msg_name = NULL,
> +		.msg_namelen = 0,
> +		.msg_control = NULL,
> +		.msg_controllen = 0,
> +		.msg_iov = (struct iovec *)vq->iov + 1,
> +		.msg_flags = MSG_DONTWAIT,
> +	};
> +	size_t len;
> +	int err;
> +	struct socket *sock = rcu_dereference(net->sock);
> +	if (!sock || !sock_writeable(sock->sk))
> +		return;
> +
> +	use_mm(net->dev.mm);
> +	mutex_lock(&vq->mutex);
> +	for (;;) {
> +		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in);
> +		if (head == vq->num)
> +			break;
> +		if (out <= 1 || in) {
> +			vq_err(vq, "Unexpected descriptor format for TX: "
> +			       "out %d, int %d\n", out, in);
> +			break;
> +		}
> +		/* Sanity check */
> +		if (vq->iov->iov_len != sizeof(struct virtio_net_hdr)) {
> +			vq_err(vq, "Unexpected header len for TX: "
> +			       "%ld expected %zd\n", vq->iov->iov_len,
> +			       sizeof(struct virtio_net_hdr));
> +			break;
> +		}
> +		/* Skip header. TODO: support TSO. */
> +		msg.msg_iovlen = out - 1;
> +		len = iov_length(vq->iov + 1, out - 1);
> +		/* TODO: Check specific error and bomb out unless ENOBUFS? */
> +		err = sock->ops->sendmsg(NULL, sock, &msg, len);
> +		if (err < 0) {
> +			vhost_discard_vq_desc(vq);
> +			break;
> +		}
> +		if (err != len)
> +			pr_err("Truncated TX packet: "
> +			       " len %d != %zd\n", err, len);
> +		vhost_add_used_and_trigger(vq, head,
> +				     len + sizeof(struct virtio_net_hdr));
> +	}
> +
> +	mutex_unlock(&vq->mutex);
> +	unuse_mm(net->dev.mm);
> +}

I guess that this is where one could plug into macvlan directly, using
sock_alloc_send_skb/memcpy_fromiovec/dev_queue_xmit directly,
instead of filling a msghdr for each, if we want to combine this
with the work I did on that.

	Arnd <><
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On Mon, Aug 10, 2009 at 09:51:18PM +0200, Arnd Bergmann wrote:
> On Monday 10 August 2009, Michael S. Tsirkin wrote:
> > 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.
> 
> Very nice, I loved reading it. It's getting rather late in my time
> zone, so this comments only on the network driver. I'll go through
> the rest tomorrow.
> 
> > @@ -293,6 +293,7 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
> >  		err = PTR_ERR(vblk->vq);
> >  		goto out_free_vblk;
> >  	}
> > +	printk(KERN_ERR "vblk->vq = %p\n", vblk->vq);
> >  
> >  	vblk->pool = mempool_create_kmalloc_pool(1,sizeof(struct virtblk_req));
> >  	if (!vblk->pool) {
> > @@ -383,6 +384,8 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
> >  	if (!err)
> >  		blk_queue_logical_block_size(vblk->disk->queue, blk_size);
> >  
> > +	printk(KERN_ERR "virtio_config_val returned %d\n", err);
> > +
> >  	add_disk(vblk->disk);
> >  	return 0;
> 
> I guess you meant to remove these before submitting.

Good catch, thanks!

> > +static void handle_tx_kick(struct work_struct *work);
> > +static void handle_rx_kick(struct work_struct *work);
> > +static void handle_tx_net(struct work_struct *work);
> > +static void handle_rx_net(struct work_struct *work);
> 
> [style] I think the code gets more readable if you reorder it
> so that you don't need forward declarations for static functions.

Right.

> > +static long vhost_net_reset_owner(struct vhost_net *n)
> > +{
> > +	struct socket *sock = NULL;
> > +	long r;
> > +	mutex_lock(&n->dev.mutex);
> > +	r = vhost_dev_check_owner(&n->dev);
> > +	if (r)
> > +		goto done;
> > +	sock = vhost_net_stop(n);
> > +	r = vhost_dev_reset_owner(&n->dev);
> > +done:
> > +	mutex_unlock(&n->dev.mutex);
> > +	if (sock)
> > +		fput(sock->file);
> > +	return r;
> > +}
> 
> what is the difference between vhost_net_reset_owner(n)
> and vhost_net_set_socket(n, -1)?

set socket to -1 will only stop the device.

reset owner will let another process take over the device.
It also needs to reset all parameters to make it safe for that
other process, so in particular the device is stopped.

I tried explaining this in the header vhost.h - does the comment
there help, or do I need to clarify it?

> > +
> > +static struct file_operations vhost_net_fops = {
> > +	.owner          = THIS_MODULE,
> > +	.release        = vhost_net_release,
> > +	.unlocked_ioctl = vhost_net_ioctl,
> > +	.open           = vhost_net_open,
> > +};
> 
> This is missing a compat_ioctl pointer. It should simply be
> 
> static long vhost_net_compat_ioctl(struct file *f,
> 			unsigned int ioctl, unsigned long arg)
> {
> 	return f, ioctl, (unsigned long)compat_ptr(arg);
> }

I had the impression that if there's no compat_ioctl,
unlocked_ioctl will get called automatically. No?

> > +/* Bits from fs/aio.c. TODO: export and use from there? */
> > +/*
> > + * use_mm
> > + *	Makes the calling kernel thread take on the specified
> > + *	mm context.
> > + *	Called by the retry thread execute retries within the
> > + *	iocb issuer's mm context, so that copy_from/to_user
> > + *	operations work seamlessly for aio.
> > + *	(Note: this routine is intended to be called only
> > + *	from a kernel thread context)
> > + */
> > +static void use_mm(struct mm_struct *mm)
> > +{
> > +	struct mm_struct *active_mm;
> > +	struct task_struct *tsk = current;
> > +
> > +	task_lock(tsk);
> > +	active_mm = tsk->active_mm;
> > +	atomic_inc(&mm->mm_count);
> > +	tsk->mm = mm;
> > +	tsk->active_mm = mm;
> > +	switch_mm(active_mm, mm, tsk);
> > +	task_unlock(tsk);
> > +
> > +	mmdrop(active_mm);
> > +}
> 
> Why do you need a kernel thread here? If the data transfer functions
> all get called from a guest intercept, shouldn't you already be
> in the right mm?

several reasons :)
- I get called under lock, so can't block
- eventfd can be passed to another process, and I won't be in guest context at all
- this also gets called outside guest context from socket poll
- vcpu is blocked while it's doing i/o. it is better to free it up
  as all the packet copying might take a while

> > +static void handle_tx(struct vhost_net *net)
> > +{
> > +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> > +	unsigned head, out, in;
> > +	struct msghdr msg = {
> > +		.msg_name = NULL,
> > +		.msg_namelen = 0,
> > +		.msg_control = NULL,
> > +		.msg_controllen = 0,
> > +		.msg_iov = (struct iovec *)vq->iov + 1,
> > +		.msg_flags = MSG_DONTWAIT,
> > +	};
> > +	size_t len;
> > +	int err;
> > +	struct socket *sock = rcu_dereference(net->sock);
> > +	if (!sock || !sock_writeable(sock->sk))
> > +		return;
> > +
> > +	use_mm(net->dev.mm);
> > +	mutex_lock(&vq->mutex);
> > +	for (;;) {
> > +		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in);
> > +		if (head == vq->num)
> > +			break;
> > +		if (out <= 1 || in) {
> > +			vq_err(vq, "Unexpected descriptor format for TX: "
> > +			       "out %d, int %d\n", out, in);
> > +			break;
> > +		}
> > +		/* Sanity check */
> > +		if (vq->iov->iov_len != sizeof(struct virtio_net_hdr)) {
> > +			vq_err(vq, "Unexpected header len for TX: "
> > +			       "%ld expected %zd\n", vq->iov->iov_len,
> > +			       sizeof(struct virtio_net_hdr));
> > +			break;
> > +		}
> > +		/* Skip header. TODO: support TSO. */
> > +		msg.msg_iovlen = out - 1;
> > +		len = iov_length(vq->iov + 1, out - 1);
> > +		/* TODO: Check specific error and bomb out unless ENOBUFS? */
> > +		err = sock->ops->sendmsg(NULL, sock, &msg, len);
> > +		if (err < 0) {
> > +			vhost_discard_vq_desc(vq);
> > +			break;
> > +		}
> > +		if (err != len)
> > +			pr_err("Truncated TX packet: "
> > +			       " len %d != %zd\n", err, len);
> > +		vhost_add_used_and_trigger(vq, head,
> > +				     len + sizeof(struct virtio_net_hdr));
> > +	}
> > +
> > +	mutex_unlock(&vq->mutex);
> > +	unuse_mm(net->dev.mm);
> > +}
> 
> I guess that this is where one could plug into macvlan directly, using
> sock_alloc_send_skb/memcpy_fromiovec/dev_queue_xmit directly,
> instead of filling a msghdr for each, if we want to combine this
> with the work I did on that.

quite possibly. Or one can just bind a raw socket to macvlan :)

> 	Arnd <><
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On Monday 10 August 2009 20:10:44 Michael S. Tsirkin wrote:
> On Mon, Aug 10, 2009 at 09:51:18PM +0200, Arnd Bergmann wrote:
> > what is the difference between vhost_net_reset_owner(n)
> > and vhost_net_set_socket(n, -1)?
> 
> set socket to -1 will only stop the device.
> 
> reset owner will let another process take over the device.
> It also needs to reset all parameters to make it safe for that
> other process, so in particular the device is stopped.

ok
 
> I tried explaining this in the header vhost.h - does the comment
> there help, or do I need to clarify it?

No, I just didn't get there yet.

> I had the impression that if there's no compat_ioctl,
> unlocked_ioctl will get called automatically. No?

It will issue a kernel warning but not call unlocked_ioctl,
so you need either a compat_ioctl method or list the numbers
in fs/compat_ioctl.c, which I try to avoid.

> > Why do you need a kernel thread here? If the data transfer functions
> > all get called from a guest intercept, shouldn't you already be
> > in the right mm?
> 
> several reasons :)
> - I get called under lock, so can't block
> - eventfd can be passed to another process, and I won't be in guest context at all
> - this also gets called outside guest context from socket poll
> - vcpu is blocked while it's doing i/o. it is better to free it up
>   as all the packet copying might take a while

Ok.

> > I guess that this is where one could plug into macvlan directly, using
> > sock_alloc_send_skb/memcpy_fromiovec/dev_queue_xmit directly,
> > instead of filling a msghdr for each, if we want to combine this
> > with the work I did on that.
> 
> quite possibly. Or one can just bind a raw socket to macvlan :)

Right, that works as well, but may get more complicated once we
try to add zero-copy or other optimizations.

	Arnd <><
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On Monday 10 August 2009, Michael S. Tsirkin wrote:

> +struct workqueue_struct *vhost_workqueue;

[nitpicking] This could be static. 

> +/* 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 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;
> +
> +	/* Last index we used. */
> +	u16 last_used_idx;
> +
> +	/* Outstanding buffers */
> +	unsigned int inflight;
> +
> +	/* Is this blocked? */
> +	bool blocked;
> +
> +	struct iovec iov[VHOST_NET_MAX_SG];
> +
> +} ____cacheline_aligned;

We discussed this before, and I still think this could be directly derived
from struct virtqueue, in the same way that vring_virtqueue is derived from
struct virtqueue. That would make it possible for simple device drivers
to use the same driver in both host and guest, similar to how Ira Snyder
used virtqueues to make virtio_net run between two hosts running the
same code [1].

Ideally, I guess you should be able to even make virtio_net work in the
host if you do that, but that could bring other complexities.

	Arnd <><

[1] http://lkml.org/lkml/2009/2/23/353
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On Wed, Aug 12, 2009 at 07:03:22PM +0200, Arnd Bergmann wrote:
> On Monday 10 August 2009, Michael S. Tsirkin wrote:
> 
> > +struct workqueue_struct *vhost_workqueue;
> 
> [nitpicking] This could be static. 
> 
> > +/* 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 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;
> > +
> > +	/* Last index we used. */
> > +	u16 last_used_idx;
> > +
> > +	/* Outstanding buffers */
> > +	unsigned int inflight;
> > +
> > +	/* Is this blocked? */
> > +	bool blocked;
> > +
> > +	struct iovec iov[VHOST_NET_MAX_SG];
> > +
> > +} ____cacheline_aligned;
> 
> We discussed this before, and I still think this could be directly derived
> from struct virtqueue, in the same way that vring_virtqueue is derived from
> struct virtqueue. That would make it possible for simple device drivers
> to use the same driver in both host and guest, similar to how Ira Snyder
> used virtqueues to make virtio_net run between two hosts running the
> same code [1].
> 
> Ideally, I guess you should be able to even make virtio_net work in the
> host if you do that, but that could bring other complexities.

I have no comments about the vhost code itself, I haven't reviewed it.

It might be interesting to try using a virtio-net in the host kernel to
communicate with the virtio-net running in the guest kernel. The lack of
a management interface is the biggest problem you will face (setting MAC
addresses, negotiating features, etc. doesn't work intuitively). Getting
the network interfaces talking is relatively easy.

Ira
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On Wed, Aug 12, 2009 at 07:03:22PM +0200, Arnd Bergmann wrote:
> On Monday 10 August 2009, Michael S. Tsirkin wrote:
> 
> > +struct workqueue_struct *vhost_workqueue;
> 
> [nitpicking] This could be static. 

Good catch. Thanks!

> > +/* 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 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;
> > +
> > +	/* Last index we used. */
> > +	u16 last_used_idx;
> > +
> > +	/* Outstanding buffers */
> > +	unsigned int inflight;
> > +
> > +	/* Is this blocked? */
> > +	bool blocked;
> > +
> > +	struct iovec iov[VHOST_NET_MAX_SG];
> > +
> > +} ____cacheline_aligned;
> 
> We discussed this before, and I still think this could be directly derived
> from struct virtqueue, in the same way that vring_virtqueue is derived from
> struct virtqueue.

I prefer keeping it simple. Much of abstraction in virtio is due to the
fact that it needs to work on top of different hardware emulations:
lguest,kvm, possibly others in the future.  vhost is always working on
real hardware, using eventfd as the interface, so it does not need that.

> That would make it possible for simple device drivers
> to use the same driver in both host and guest,

I don't think so. For example, there's a callback field that gets
invoked in guest when buffers are consumed.  It could be overloaded to
mean "buffers are available" in host but you never handle both
situations in the same way, so what's the point?

> similar to how Ira Snyder used virtqueues to make virtio_net run
> between two hosts running the same code [1].
> Ideally, I guess you should be able to even make virtio_net work in the
> host if you do that, but that could bring other complexities.
> 
> 	Arnd <><
> 
> [1] http://lkml.org/lkml/2009/2/23/353

As I pointed out earlier, most code in virtio net is asymmetrical: guest
provides buffers, host consumes them.  Possibly, one could use virtio
rings in a symmetrical way, but support of existing guest virtio net
means there's almost no shared code.

On Wed, Aug 12, 2009 at 10:19:22AM -0700, Ira W. Snyder wrote:
> On Wed, Aug 12, 2009 at 07:03:22PM +0200, Arnd Bergmann wrote:
> > On Monday 10 August 2009, Michael S. Tsirkin wrote:
> > 
> > > +struct workqueue_struct *vhost_workqueue;
> > 
> > [nitpicking] This could be static. 
> > 
> > > +/* 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 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;
> > > +
> > > +	/* Last index we used. */
> > > +	u16 last_used_idx;
> > > +
> > > +	/* Outstanding buffers */
> > > +	unsigned int inflight;
> > > +
> > > +	/* Is this blocked? */
> > > +	bool blocked;
> > > +
> > > +	struct iovec iov[VHOST_NET_MAX_SG];
> > > +
> > > +} ____cacheline_aligned;
> > 
> > We discussed this before, and I still think this could be directly derived
> > from struct virtqueue, in the same way that vring_virtqueue is derived from
> > struct virtqueue. That would make it possible for simple device drivers
> > to use the same driver in both host and guest, similar to how Ira Snyder
> > used virtqueues to make virtio_net run between two hosts running the
> > same code [1].
> > 
> > Ideally, I guess you should be able to even make virtio_net work in the
> > host if you do that, but that could bring other complexities.
> 
> I have no comments about the vhost code itself, I haven't reviewed it.
> 
> It might be interesting to try using a virtio-net in the host kernel to
> communicate with the virtio-net running in the guest kernel. The lack of
> a management interface is the biggest problem you will face (setting MAC
> addresses, negotiating features, etc. doesn't work intuitively).

That was one of the reasons I decided to move most of code out to
userspace. My kernel driver only handles datapath,
it's much smaller than virtio net.

> Getting
> the network interfaces talking is relatively easy.
> 
> Ira

Tried this, but
- guest memory isn't pinned, so copy_to_user
  to access it, errors need to be handled in a sane way
- used/available roles are reversed
- kick/interrupt roles are reversed

So most of the code then looks like

	if (host) {
	} else {
	}
	return


The only common part is walking the descriptor list,
but that's like 10 lines of code.

At which point it's better to keep host/guest code separate, IMO.

On Wed, Aug 12, 2009 at 08:31:04PM +0300, Michael S. Tsirkin wrote:
> On Wed, Aug 12, 2009 at 10:19:22AM -0700, Ira W. Snyder wrote:

[ snip out code ]

> > > 
> > > We discussed this before, and I still think this could be directly derived
> > > from struct virtqueue, in the same way that vring_virtqueue is derived from
> > > struct virtqueue. That would make it possible for simple device drivers
> > > to use the same driver in both host and guest, similar to how Ira Snyder
> > > used virtqueues to make virtio_net run between two hosts running the
> > > same code [1].
> > > 
> > > Ideally, I guess you should be able to even make virtio_net work in the
> > > host if you do that, but that could bring other complexities.
> > 
> > I have no comments about the vhost code itself, I haven't reviewed it.
> > 
> > It might be interesting to try using a virtio-net in the host kernel to
> > communicate with the virtio-net running in the guest kernel. The lack of
> > a management interface is the biggest problem you will face (setting MAC
> > addresses, negotiating features, etc. doesn't work intuitively).
> 
> That was one of the reasons I decided to move most of code out to
> userspace. My kernel driver only handles datapath,
> it's much smaller than virtio net.
> 
> > Getting
> > the network interfaces talking is relatively easy.
> > 
> > Ira
> 
> Tried this, but
> - guest memory isn't pinned, so copy_to_user
>   to access it, errors need to be handled in a sane way
> - used/available roles are reversed
> - kick/interrupt roles are reversed
> 
> So most of the code then looks like
> 
> 	if (host) {
> 	} else {
> 	}
> 	return
> 
> 
> The only common part is walking the descriptor list,
> but that's like 10 lines of code.
> 
> At which point it's better to keep host/guest code separate, IMO.
> 

Ok, that makes sense. Let me see if I understand the concept of the
driver. Here's a picture of what makes sense to me:

guest system
---------------------------------
| userspace applications        |
---------------------------------
| kernel network stack          |
---------------------------------
| virtio-net                    |
---------------------------------
| transport (virtio-ring, etc.) |
---------------------------------
               |
               |
---------------------------------
| transport (virtio-ring, etc.) |
---------------------------------
| some driver (maybe vhost?)    | <-- [1]
---------------------------------
| kernel network stack          |
---------------------------------
host system

From the host's network stack, packets can be forwarded out to the
physical network, or be consumed by a normal userspace application on
the host. Just as if this were any other network interface.

In my patch, [1] was the virtio-net driver, completely unmodified.

So, does this patch accomplish the above diagram? If so, why the
copy_to_user(), etc? Maybe I'm confusing this with my system, where the
"guest" is another physical system, separated by the PCI bus.

Ira
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On Wednesday 12 August 2009, Michael S. Tsirkin wrote:
> On Wed, Aug 12, 2009 at 07:03:22PM +0200, Arnd Bergmann wrote:
> > We discussed this before, and I still think this could be directly derived
> > from struct virtqueue, in the same way that vring_virtqueue is derived from
> > struct virtqueue.
> 
> I prefer keeping it simple. Much of abstraction in virtio is due to the
> fact that it needs to work on top of different hardware emulations:
> lguest,kvm, possibly others in the future.  vhost is always working on
> real hardware, using eventfd as the interface, so it does not need that.

Well, that was my point: virtio can already work on a number of abstractions,
so adding one more for vhost should not be too hard.

> > That would make it possible for simple device drivers
> > to use the same driver in both host and guest,
> 
> I don't think so. For example, there's a callback field that gets
> invoked in guest when buffers are consumed.  It could be overloaded to
> mean "buffers are available" in host but you never handle both
> situations in the same way, so what's the point?
>
...
> 
> As I pointed out earlier, most code in virtio net is asymmetrical: guest
> provides buffers, host consumes them.  Possibly, one could use virtio
> rings in a symmetrical way, but support of existing guest virtio net
> means there's almost no shared code.

The trick is to swap the virtqueues instead. virtio-net is actually
mostly symmetric in just the same way that the physical wires on a
twisted pair ethernet are symmetric (I like how that analogy fits).

virtio_net kicks the transmit virtqueue when it has data and
it kicks the receive queue when it has empty buffers to fill,
and it has callbacks when the two are done. You can do the
same in both the guest and the host, but then the guests input
virtqueue is the hosts output virtqueue and vice versa.

Once a virtqueue got kicked from both sides, the vhost_virtqueue
implementation between the two only needs to do a copy_from_user
or copy_to_user (possibly from a thread if it is in atomic context)
and then call the two callback functions. This is basically the
same thing you do already, except that you use slightly different
names for the components.

	Arnd <><
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Michael S. Tsirkin wrote:
>>
>> We discussed this before, and I still think this could be directly derived
>> from struct virtqueue, in the same way that vring_virtqueue is derived from
>> struct virtqueue.
>>     
>
> I prefer keeping it simple. Much of abstraction in virtio is due to the
> fact that it needs to work on top of different hardware emulations:
> lguest,kvm, possibly others in the future.  vhost is always working on
> real hardware, using eventfd as the interface, so it does not need that.
>   

Actually, vhost may not always be limited to real hardware.

We may on day use vhost as the basis of a driver domain.  There's quite 
a lot of interest in this for networking.

At any rate, I'd like to see performance results before we consider 
trying to reuse virtio code.

Regards,

Anthony Liguori
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Arnd Bergmann wrote:
>> As I pointed out earlier, most code in virtio net is asymmetrical: guest
>> provides buffers, host consumes them.  Possibly, one could use virtio
>> rings in a symmetrical way, but support of existing guest virtio net
>> means there's almost no shared code.
>>     
>
> The trick is to swap the virtqueues instead. virtio-net is actually
> mostly symmetric in just the same way that the physical wires on a
> twisted pair ethernet are symmetric (I like how that analogy fits).
>   

It's already been done between two guests.  See 
http://article.gmane.org/gmane.linux.kernel.virtualization/5423

Regards,

Anthony Liguori
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On Mon, Aug 10, 2009 at 09:53:40PM +0300, Michael S. Tsirkin wrote:
> 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)
> - 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 backend 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
> tun-like device. In this version I only support raw socket as a backend,
> which can be bound to e.g. SR IOV, or to macvlan device.  Backend is
> also configured by userspace, including vlan/mac etc.
> 
> Status:
> This works for me, and I haven't see any crashes.
> I have not run any benchmarks yet, compared to userspace, I expect to
> see improved latency (as I save up to 4 system calls per packet) but not
> yet bandwidth/CPU (as TSO and interrupt mitigation are not yet supported).
> 
> Features that I plan to look at in the future:
> - TSO
> - interrupt mitigation
> - zero copy

Much better -- a couple of documentation nits below.

							Thanx, Paul

> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> 
> ---
>  MAINTAINERS                |   10 +
>  arch/x86/kvm/Kconfig       |    1 +
>  drivers/Makefile           |    1 +
>  drivers/block/virtio_blk.c |    3 +
>  drivers/vhost/Kconfig      |   11 +
>  drivers/vhost/Makefile     |    2 +
>  drivers/vhost/net.c        |  462 ++++++++++++++++++++++++++++++
>  drivers/vhost/vhost.c      |  663 ++++++++++++++++++++++++++++++++++++++++++++
>  drivers/vhost/vhost.h      |  108 +++++++
>  include/linux/Kbuild       |    1 +
>  include/linux/miscdevice.h |    1 +
>  include/linux/vhost.h      |  100 +++++++
>  12 files changed, 1363 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
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index ebc2691..eb0c1da 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -6312,6 +6312,16 @@ S:	Maintained
>  F:	Documentation/filesystems/vfat.txt
>  F:	fs/fat/
> 
> +VIRTIO HOST (VHOST)
> +P:	Michael S. Tsirkin
> +M:	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
>  P:	Roger Luethi
>  M:	rl@hellgate.ch
> 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 bc4205d..1551ae1 100644
> --- a/drivers/Makefile
> +++ b/drivers/Makefile
> @@ -105,6 +105,7 @@ 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/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index aa1a3d5..42e61b0 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -293,6 +293,7 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
>  		err = PTR_ERR(vblk->vq);
>  		goto out_free_vblk;
>  	}
> +	printk(KERN_ERR "vblk->vq = %p\n", vblk->vq);
> 
>  	vblk->pool = mempool_create_kmalloc_pool(1,sizeof(struct virtblk_req));
>  	if (!vblk->pool) {
> @@ -383,6 +384,8 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
>  	if (!err)
>  		blk_queue_logical_block_size(vblk->disk->queue, blk_size);
> 
> +	printk(KERN_ERR "virtio_config_val returned %d\n", err);
> +
>  	add_disk(vblk->disk);
>  	return 0;
> 
> 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..fc3359b
> --- /dev/null
> +++ b/drivers/vhost/net.c
> @@ -0,0 +1,462 @@
> +/* 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/eventfd.h>
> +#include <linux/vhost.h>
> +#include <linux/virtio_net.h>
> +#include <linux/mm.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 <net/sock.h>
> +
> +#include <asm/mmu_context.h>
> +
> +#include "vhost.h"
> +
> +enum {
> +	VHOST_NET_VQ_RX = 0,
> +	VHOST_NET_VQ_TX = 1,
> +	VHOST_NET_VQ_MAX = 2,
> +};
> +
> +struct vhost_net {
> +	struct vhost_dev dev;
> +	struct vhost_virtqueue vqs[VHOST_NET_VQ_MAX];
> +	/* We use a kind of RCU to access sock pointer.
> +	 * All readers access it from workqueue,
> +	 * which makes it possible to flush the workqueue
> +	 * instead of synchronize_rcu. Therefore readers
> +	 * do not need rcu_read_lock/rcu_read_unlock.

How about something like "Therefore the beginning of workqueue
execution acts as rcu_read_lock() and the end of workqueue execution
acts as rcu_read_lock()"?

It would also be good to add comments to the workqueue functions
themselves saying that they act as read-side critical sections for
your kind of RCU.

							Thanx, Paul

> +	 * Writers use device mutex. */
> +	struct socket *sock;
> +	struct vhost_poll poll[VHOST_NET_VQ_MAX];
> +};
> +
> +static void handle_tx_kick(struct work_struct *work);
> +static void handle_rx_kick(struct work_struct *work);
> +static void handle_tx_net(struct work_struct *work);
> +static void handle_rx_net(struct work_struct *work);
> +
> +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);
> +	return 0;
> +}
> +
> +static struct socket *vhost_net_stop(struct vhost_net *n)
> +{
> +	struct socket *sock = n->sock;
> +	rcu_assign_pointer(n->sock, NULL);
> +	if (sock) {
> +		vhost_poll_flush(n->poll + VHOST_NET_VQ_TX);
> +		vhost_poll_flush(n->poll + VHOST_NET_VQ_RX);
> +	}
> +	return sock;
> +}
> +
> +static int vhost_net_release(struct inode *inode, struct file *f)
> +{
> +	struct vhost_net *n = f->private_data;
> +	struct socket *sock;
> +
> +	sock = vhost_net_stop(n);
> +	vhost_dev_cleanup(&n->dev);
> +	if (sock)
> +		fput(sock->file);
> +	kfree(n);
> +	return 0;
> +}
> +
> +static long vhost_net_set_socket(struct vhost_net *n, int fd)
> +{
> +	struct {
> +		struct sockaddr_ll sa;
> +		char  buf[MAX_ADDR_LEN];
> +	} uaddr;
> +	struct socket *sock, *oldsock = NULL;
> +	int uaddr_len = sizeof uaddr, r;
> +
> +	mutex_lock(&n->dev.mutex);
> +	r = vhost_dev_check_owner(&n->dev);
> +	if (r)
> +		goto done;
> +
> +	if (fd == -1) {
> +		/* Disconnect from socket and device. */
> +		oldsock = vhost_net_stop(n);
> +		goto done;
> +	}
> +	
> +	sock = sockfd_lookup(fd, &r);
> +	if (!sock) {
> +		r = -ENOTSOCK;
> +		goto done;
> +	}
> +
> +	/* Parameter checking */
> +	if (sock->sk->sk_type != SOCK_RAW) {
> +		r = -ESOCKTNOSUPPORT;
> +		goto done;
> +	}
> +
> +	r = sock->ops->getname(sock, (struct sockaddr *)&uaddr.sa,
> +			       &uaddr_len, 0);
> +	if (r)
> +		goto done;
> +
> +	if (uaddr.sa.sll_family != AF_PACKET) {
> +		r = -EPFNOSUPPORT;
> +		goto done;
> +	}
> +
> +	/* start polling new socket */
> +	if (sock == oldsock)
> +		goto done;
> +
> +	if (oldsock) {
> +		vhost_poll_stop(n->poll + VHOST_NET_VQ_TX);
> +		vhost_poll_stop(n->poll + VHOST_NET_VQ_RX);
> +	}
> +	oldsock = n->sock;
> +	rcu_assign_pointer(n->sock, sock);
> +	vhost_poll_start(n->poll + VHOST_NET_VQ_TX, sock->file);
> +	vhost_poll_start(n->poll + VHOST_NET_VQ_RX, sock->file);
> +done:
> +	mutex_unlock(&n->dev.mutex);
> +	if (oldsock) {
> +		vhost_poll_flush(n->poll + VHOST_NET_VQ_TX);
> +		vhost_poll_flush(n->poll + VHOST_NET_VQ_RX);
> +		vhost_poll_flush(&n->dev.vqs[VHOST_NET_VQ_TX].poll);
> +		vhost_poll_flush(&n->dev.vqs[VHOST_NET_VQ_RX].poll);
> +		fput(oldsock->file);
> +	}
> +	return r;
> +}
> +
> +static long vhost_net_reset_owner(struct vhost_net *n)
> +{
> +	struct socket *sock = NULL;
> +	long r;
> +	mutex_lock(&n->dev.mutex);
> +	r = vhost_dev_check_owner(&n->dev);
> +	if (r)
> +		goto done;
> +	sock = vhost_net_stop(n);
> +	r = vhost_dev_reset_owner(&n->dev);
> +done:
> +	mutex_unlock(&n->dev.mutex);
> +	if (sock)
> +		fput(sock->file);
> +	return r;
> +}
> +
> +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;
> +	int __user *fdp = argp;
> +	u32 features;
> +	int fd, r;
> +	switch (ioctl) {
> +	case VHOST_NET_SET_SOCKET:
> +		r = get_user(fd, fdp);
> +		if (r < 0)
> +			return r;
> +		return vhost_net_set_socket(n, fd);
> +	case VHOST_GET_FEATURES:
> +		/* No features for now */
> +		features = 0;
> +		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)
> +			return -EOPNOTSUPP;
> +		return 0;
> +	case VHOST_RESET_OWNER:
> +		return vhost_net_reset_owner(n);
> +	default:
> +		return vhost_dev_ioctl(&n->dev, ioctl, arg);
> +	}
> +}
> +
> +static struct file_operations vhost_net_fops = {
> +	.owner          = THIS_MODULE,
> +	.release        = vhost_net_release,
> +	.unlocked_ioctl = vhost_net_ioctl,
> +	.open           = vhost_net_open,
> +};
> +
> +static struct miscdevice vhost_net_misc = {
> +	VHOST_NET_MINOR,
> +	"vhost-net",
> +	&vhost_net_fops,
> +};
> +
> +/* Bits from fs/aio.c. TODO: export and use from there? */
> +/*
> + * use_mm
> + *	Makes the calling kernel thread take on the specified
> + *	mm context.
> + *	Called by the retry thread execute retries within the
> + *	iocb issuer's mm context, so that copy_from/to_user
> + *	operations work seamlessly for aio.
> + *	(Note: this routine is intended to be called only
> + *	from a kernel thread context)
> + */
> +static void use_mm(struct mm_struct *mm)
> +{
> +	struct mm_struct *active_mm;
> +	struct task_struct *tsk = current;
> +
> +	task_lock(tsk);
> +	active_mm = tsk->active_mm;
> +	atomic_inc(&mm->mm_count);
> +	tsk->mm = mm;
> +	tsk->active_mm = mm;
> +	switch_mm(active_mm, mm, tsk);
> +	task_unlock(tsk);
> +
> +	mmdrop(active_mm);
> +}
> +
> +/*
> + * unuse_mm
> + *	Reverses the effect of use_mm, i.e. releases the
> + *	specified mm context which was earlier taken on
> + *	by the calling kernel thread
> + *	(Note: this routine is intended to be called only
> + *	from a kernel thread context)
> + */
> +static void unuse_mm(struct mm_struct *mm)
> +{
> +	struct task_struct *tsk = current;
> +
> +	task_lock(tsk);
> +	tsk->mm = NULL;
> +	/* active_mm is still 'mm' */
> +	enter_lazy_tlb(mm, tsk);
> +	task_unlock(tsk);
> +}
> +
> +static void handle_tx(struct vhost_net *net)
> +{
> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> +	unsigned head, out, in;
> +	struct msghdr msg = {
> +		.msg_name = NULL,
> +		.msg_namelen = 0,
> +		.msg_control = NULL,
> +		.msg_controllen = 0,
> +		.msg_iov = (struct iovec *)vq->iov + 1,
> +		.msg_flags = MSG_DONTWAIT,
> +	};
> +	size_t len;
> +	int err;
> +	struct socket *sock = rcu_dereference(net->sock);
> +	if (!sock || !sock_writeable(sock->sk))
> +		return;
> +
> +	use_mm(net->dev.mm);
> +	mutex_lock(&vq->mutex);
> +	for (;;) {
> +		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in);
> +		if (head == vq->num)
> +			break;
> +		if (out <= 1 || in) {
> +			vq_err(vq, "Unexpected descriptor format for TX: "
> +			       "out %d, int %d\n", out, in);
> +			break;
> +		}
> +		/* Sanity check */
> +		if (vq->iov->iov_len != sizeof(struct virtio_net_hdr)) {
> +			vq_err(vq, "Unexpected header len for TX: "
> +			       "%ld expected %zd\n", vq->iov->iov_len,
> +			       sizeof(struct virtio_net_hdr));
> +			break;
> +		}
> +		/* Skip header. TODO: support TSO. */
> +		msg.msg_iovlen = out - 1;
> +		len = iov_length(vq->iov + 1, out - 1);
> +		/* TODO: Check specific error and bomb out unless ENOBUFS? */
> +		err = sock->ops->sendmsg(NULL, sock, &msg, len);
> +		if (err < 0) {
> +			vhost_discard_vq_desc(vq);
> +			break;
> +		}
> +		if (err != len)
> +			pr_err("Truncated TX packet: "
> +			       " len %d != %zd\n", err, len);
> +		vhost_add_used_and_trigger(vq, head,
> +				     len + sizeof(struct virtio_net_hdr));
> +	}
> +
> +	mutex_unlock(&vq->mutex);
> +	unuse_mm(net->dev.mm);
> +}
> +
> +static void handle_rx(struct vhost_net *net)
> +{
> +	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_RX];
> +	unsigned head, out, in;
> +	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 + 1,
> +		.msg_flags = MSG_DONTWAIT,
> +	};
> +
> +	struct virtio_net_hdr hdr = {
> +		.flags = 0,
> +		.gso_type = VIRTIO_NET_HDR_GSO_NONE
> +	};
> +
> +	size_t len;
> +	int err;
> +	struct socket *sock = rcu_dereference(net->sock);
> +	if (!sock || skb_queue_empty(&sock->sk->sk_receive_queue))
> +		return;
> +
> +	use_mm(net->dev.mm);
> +	mutex_lock(&vq->mutex);
> +
> +	for (;;) {
> +		head = vhost_get_vq_desc(&net->dev, vq, vq->iov, &out, &in);
> +		if (head == vq->num)
> +			break;
> +		if (in <= 1 || out) {
> +			vq_err(vq, "Unexpected descriptor format for RX: out %d, int %d\n",
> +			       out, in);
> +			break;
> +		}
> +		/* Sanity check */
> +		if (vq->iov->iov_len != sizeof(struct virtio_net_hdr)) {
> +			vq_err(vq, "Unexpected header len for RX: %ld expected %zd\n",
> +			       vq->iov->iov_len, sizeof(struct virtio_net_hdr));
> +			break;
> +		}
> +		/* Skip header. TODO: support TSO/mergeable rx buffers. */
> +		msg.msg_iovlen = in - 1;
> +		len = iov_length(vq->iov + 1, in - 1);
> +		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 = copy_to_user(vq->iov->iov_base, &hdr, sizeof hdr);
> +		if (err) {
> +			vq_err(vq, "Unable to write vnet_hdr at addr %p: %d\n",
> +			       vq->iov->iov_base, err);
> +			break;
> +		}
> +		vhost_add_used_and_trigger(vq, head, len + sizeof hdr);
> +	}
> +
> +	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);
> +}
> +
> +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..6178ec1
> --- /dev/null
> +++ b/drivers/vhost/vhost.c
> @@ -0,0 +1,663 @@
> +/* 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/net.h>
> +#include <linux/if_packet.h>
> +#include <linux/if_arp.h>
> +
> +#include <net/sock.h>
> +
> +#include <asm/mmu_context.h>
> +
> +#include "vhost.h"
> +
> +enum {
> +	VHOST_MEMORY_MAX_NREGIONS = 64,
> +};
> +
> +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 user 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);
> +}
> +
> +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;
> +}
> +
> +/* User should have device mutex */
> +long vhost_dev_check_owner(struct vhost_dev *dev)
> +{
> +	return dev->mm == current->mm ? 0 : -EPERM;
> +}
> +
> +/* User should have device mutex */
> +static long vhost_dev_set_owner(struct vhost_dev *dev)
> +{
> +	if (dev->mm)
> +		return -EBUSY;
> +	dev->mm = get_task_mm(current);
> +	return 0;
> +}
> +
> +/* User 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 = kmalloc(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;
> +}
> +
> +/* User 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;
> +	}
> +	/* 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)
> +{
> +	u16 flags = 0;
> +	int r = put_user(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->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)(long)a.user_addr != a.user_addr) {
> +			r = -EFAULT;
> +			break;
> +		}
> +		vq->desc = (void __user *)(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)(long)a.user_addr != a.user_addr) {
> +			r = -EFAULT;
> +			break;
> +		}
> +		vq->avail = (void __user *)(long)a.user_addr;
> +		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)(long)a.user_addr != a.user_addr) {
> +			r = -EFAULT;
> +			break;
> +		}
> +		vq->used = (void __user *)(long)a.user_addr;
> +		r = init_used(vq);
> +		if (r)
> +			break;
> +		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 = -ENOTTY;
> +	}
> +
> +	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 0;
> +}
> +
> +long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, unsigned long arg)
> +{
> +	void __user *argp = (void __user *)arg;
> +	long r;
> +
> +	mutex_lock(&d->mutex);
> +	if (ioctl == VHOST_SET_OWNER) {
> +		r = vhost_dev_set_owner(d);
> +		goto done;
> +	}
> +
> +	r = vhost_dev_check_owner(d);
> +	if (r)
> +		goto done;
> +		
> +	switch (ioctl) {
> +	case VHOST_SET_MEM_TABLE:
> +		r = vhost_set_memory(d, argp);
> +		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;
> +}
> +
> +/* FIXME: this does not handle a region that spans multiple
> + * address/len pairs */
> +int translate_desc(struct vhost_dev *dev, u64 addr, u32 len,
> +		   struct iovec iov[], int iov_count, int iov_size,
> +		   unsigned *num)
> +{
> +	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 (*num + iov_count >= iov_size) {
> +			ret = -ENOBUFS;
> +			break;
> +		}
> +		reg = find_region(mem, addr, len);
> +		if (!reg) {
> +			ret = -EFAULT;
> +			break;
> +		}
> +		_iov = iov + iov_count + *num;
> +		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;
> +		++*num;
> +	}
> +	
> +	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 vq->vring.num if we're
> + * at the end. */
> +static unsigned next_desc(struct vhost_virtqueue *vq, 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 vq->num;
> +
> +	/* 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();
> +
> +	if (next >= vq->num) {
> +		vq_err(vq, "Desc next is %u > %u", next, vq->num);
> +		return vq->num;
> +	}
> +
> +	return next;
> +}
> +
> +/* 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 vring_desc desc;
> +	unsigned int i, head;
> +	u16 last_avail_idx, idx;
> +
> +	/* Check it isn't doing very strange things with descriptor numbers. */
> +	last_avail_idx = vq->last_avail_idx;
> +	if (get_user(idx, &vq->avail->idx)) {
> +		vq_err(vq, "Failed to access avail idx at %p\n",
> +		       &vq->avail->idx);
> +		return vq->num;
> +	}
> +	
> +	if ((u16)(idx - last_avail_idx) > vq->num) {
> +		vq_err(vq, "Guest moved used index from %u to %u",
> +		       last_avail_idx, idx);
> +		return vq->num;
> +	}
> +
> +	/* If there's nothing new since last we looked, return invalid. */
> +	if (idx == last_avail_idx)
> +		return vq->num;
> +
> +	/* 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",
> +		       idx, &vq->avail->ring[last_avail_idx % vq->num]);
> +		return vq->num;
> +	}
> +
> +	/* If their number is silly, that's a fatal mistake. */
> +	if (head >= vq->num) {
> +		vq_err(vq, "Guest says index %u > %u is available",
> +		       head, vq->num);
> +		return vq->num;
> +	}
> +
> +	vq->last_avail_idx++;
> +
> +	/* When we start there are none of either input nor output. */
> +	*out_num = *in_num = 0;
> +
> +	i = head;
> +	do {
> +		unsigned *num;
> +		unsigned iov_count;
> +		if (copy_from_user(&desc, vq->desc + i, sizeof desc)) {
> +			vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
> +			       i, vq->desc + i);
> +			return vq->num;
> +		}
> +		/* If this is an input descriptor, increment that count. */
> +		if (desc.flags & VRING_DESC_F_WRITE) {
> +			num = in_num;
> +			iov_count = *out_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;
> +			}
> +			num = out_num;
> +			iov_count = *in_num;
> +		}
> +		if (translate_desc(dev, desc.addr, desc.len, iov, iov_count,
> +				   VHOST_NET_MAX_SG, num)) {
> +			vq_err(vq, "Failed to translate descriptor: idx %d\n",
> +			       i);
> +			return vq->num;
> +		}
> +
> +		/* If we've got too many, that implies a descriptor loop. */
> +		if (*out_num + *in_num > vq->num) {
> +			vq_err(vq, "Looped descriptor: idx %d\n", i);
> +			return vq->num;
> +		}
> +	} while ((i = next_desc(vq, &desc)) != vq->num);
> +
> +	vq->inflight++;
> +	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--;
> +	vq->inflight--;
> +}
> +
> +/* 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;
> +	}
> +	vq->last_used_idx++;
> +	vq->inflight--;
> +	return 0;
> +}
> +
> +/* This actually sends the interrupt for this virtqueue */
> +void vhost_trigger_irq(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->inflight)
> +		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_virtqueue *vq,
> +				unsigned int head, int len)
> +{
> +	vhost_add_used(vq, head, len);
> +	vhost_trigger_irq(vq);
> +}
> +
> +int vhost_init(void)
> +{
> +	vhost_workqueue = create_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..7f7ffcd
> --- /dev/null
> +++ b/drivers/vhost/vhost.h
> @@ -0,0 +1,108 @@
> +#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>
> +
> +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);
> +
> +/* 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 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;
> +
> +	/* Last index we used. */
> +	u16 last_used_idx;
> +
> +	/* Outstanding buffers */
> +	unsigned int inflight;
> +
> +	/* Is this blocked? */
> +	bool blocked;
> +
> +	struct iovec iov[VHOST_NET_MAX_SG];
> +
> +} ____cacheline_aligned;
> +
> +struct vhost_dev {
> +	/* Readers use RCU to access memory table pointer.
> +	 * Writers use mutex below.*/
> +	struct vhost_memory *memory;
> +	struct mm_struct *mm;
> +	struct vhost_virtqueue *vqs;
> +	int nvqs;
> +	struct mutex mutex;
> +};
> +
> +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);
> +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_virtqueue *);
> +void vhost_add_used_and_trigger(struct vhost_virtqueue *,
> +				unsigned int head, int len);
> +
> +int vhost_init(void);
> +void vhost_cleanup(void);
> +
> +#define vq_err(vq, fmt, ...) do {                                  \
> +		printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__);       \
> +		if ((vq)->error_ctx)                               \
> +				eventfd_signal((vq)->error_ctx, 1);\
> +	} while (0)
> +
> +#endif
> diff --git a/include/linux/Kbuild b/include/linux/Kbuild
> index dec2f18..975df9a 100644
> --- a/include/linux/Kbuild
> +++ b/include/linux/Kbuild
> @@ -360,6 +360,7 @@ 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 0521177..781a8bb 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..9ec6d5f
> --- /dev/null
> +++ b/include/linux/vhost.h
> @@ -0,0 +1,100 @@
> +#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;
> +};
> +
> +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 padding; /* read/write protection? */
> +};
> +
> +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 must be zero. */
> +#define VHOST_GET_FEATURES	_IOR(VHOST_VIRTIO, 0x00, __u32)
> +#define VHOST_ACK_FEATURES	_IOW(VHOST_VIRTIO, 0x00, __u32)
> +
> +/* 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)
> +
> +/* 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 */
> +#define VHOST_SET_VRING_USED _IOW(VHOST_VIRTIO, 0x12, struct vhost_vring_addr)
> +/* Available structure address */
> +#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)
> +
> +/* 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 device to a raw socket. The socket must be already
> + * bound to an ethernet device, this device will be used for transmit.
> + * Pass -1 to unbind from the socket and the transmit device.
> + * This can be used to stop the device (e.g. for migration). */
> +#define VHOST_NET_SET_SOCKET _IOW(VHOST_VIRTIO, 0x30, int)
> +
> +#endif
> -- 
> 1.6.2.5
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On Wed, Aug 12, 2009 at 10:48:21AM -0700, Ira W. Snyder wrote:
> On Wed, Aug 12, 2009 at 08:31:04PM +0300, Michael S. Tsirkin wrote:
> > On Wed, Aug 12, 2009 at 10:19:22AM -0700, Ira W. Snyder wrote:
> 
> [ snip out code ]
> 
> > > > 
> > > > We discussed this before, and I still think this could be directly derived
> > > > from struct virtqueue, in the same way that vring_virtqueue is derived from
> > > > struct virtqueue. That would make it possible for simple device drivers
> > > > to use the same driver in both host and guest, similar to how Ira Snyder
> > > > used virtqueues to make virtio_net run between two hosts running the
> > > > same code [1].
> > > > 
> > > > Ideally, I guess you should be able to even make virtio_net work in the
> > > > host if you do that, but that could bring other complexities.
> > > 
> > > I have no comments about the vhost code itself, I haven't reviewed it.
> > > 
> > > It might be interesting to try using a virtio-net in the host kernel to
> > > communicate with the virtio-net running in the guest kernel. The lack of
> > > a management interface is the biggest problem you will face (setting MAC
> > > addresses, negotiating features, etc. doesn't work intuitively).
> > 
> > That was one of the reasons I decided to move most of code out to
> > userspace. My kernel driver only handles datapath,
> > it's much smaller than virtio net.
> > 
> > > Getting
> > > the network interfaces talking is relatively easy.
> > > 
> > > Ira
> > 
> > Tried this, but
> > - guest memory isn't pinned, so copy_to_user
> >   to access it, errors need to be handled in a sane way
> > - used/available roles are reversed
> > - kick/interrupt roles are reversed
> > 
> > So most of the code then looks like
> > 
> > 	if (host) {
> > 	} else {
> > 	}
> > 	return
> > 
> > 
> > The only common part is walking the descriptor list,
> > but that's like 10 lines of code.
> > 
> > At which point it's better to keep host/guest code separate, IMO.
> > 
> 
> Ok, that makes sense. Let me see if I understand the concept of the
> driver. Here's a picture of what makes sense to me:
> 
> guest system
> ---------------------------------
> | userspace applications        |
> ---------------------------------
> | kernel network stack          |
> ---------------------------------
> | virtio-net                    |
> ---------------------------------
> | transport (virtio-ring, etc.) |
> ---------------------------------
>                |
>                |
> ---------------------------------
> | transport (virtio-ring, etc.) |
> ---------------------------------
> | some driver (maybe vhost?)    | <-- [1]
> ---------------------------------
> | kernel network stack          |
> ---------------------------------
> host system
> 
> >From the host's network stack, packets can be forwarded out to the
> physical network, or be consumed by a normal userspace application on
> the host. Just as if this were any other network interface.
> 
> In my patch, [1] was the virtio-net driver, completely unmodified.
> 
> So, does this patch accomplish the above diagram?

Not exactly. vhost passes packets to a physical device,
through a raw socket, not into host network stack.

> If so, why the copy_to_user(), etc?

Guest memory is not pinned. Memory access needs to go through
translation process, could cause page faults, etc.

> Maybe I'm confusing this with my system, where the
> "guest" is another physical system, separated by the PCI bus.
> 
> Ira

Yes, that's different.
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On Wed, Aug 12, 2009 at 07:59:47PM +0200, Arnd Bergmann wrote:
> The trick is to swap the virtqueues instead. virtio-net is actually
> mostly symmetric in just the same way that the physical wires on a
> twisted pair ethernet are symmetric (I like how that analogy fits).

You need to really squint hard for it to look symmetric.

For example, for RX, virtio allocates an skb, puts a descriptor on a
ring and waits for host to fill it in. Host system can not do the same:
guest does not have access to host memory.

You can do a copy in transport to hide this fact, but it will kill
performance.

On Wed, Aug 12, 2009 at 02:27:31PM -0500, Anthony Liguori wrote:
> Arnd Bergmann wrote:
>>> As I pointed out earlier, most code in virtio net is asymmetrical: guest
>>> provides buffers, host consumes them.  Possibly, one could use virtio
>>> rings in a symmetrical way, but support of existing guest virtio net
>>> means there's almost no shared code.
>>>     
>>
>> The trick is to swap the virtqueues instead. virtio-net is actually
>> mostly symmetric in just the same way that the physical wires on a
>> twisted pair ethernet are symmetric (I like how that analogy fits).
>>   
>
> It's already been done between two guests.  See  
> http://article.gmane.org/gmane.linux.kernel.virtualization/5423
>
> Regards,
>
> Anthony Liguori

Yes, this works by copying data (see PATCH 5/5).  Another possibility is
page flipping.  Either will kill performance.

On Wed, Aug 12, 2009 at 02:22:38PM -0500, Anthony Liguori wrote:
> Michael S. Tsirkin wrote:
>>>
>>> We discussed this before, and I still think this could be directly derived
>>> from struct virtqueue, in the same way that vring_virtqueue is derived from
>>> struct virtqueue.
>>>     
>>
>> I prefer keeping it simple. Much of abstraction in virtio is due to the
>> fact that it needs to work on top of different hardware emulations:
>> lguest,kvm, possibly others in the future.  vhost is always working on
>> real hardware, using eventfd as the interface, so it does not need that.
>>   
>
> Actually, vhost may not always be limited to real hardware.

Yes, any ethernet device will do. What I mean is that vhost does not
deal with emulation at all. All setup is done in userspace.

>
> We may on day use vhost as the basis of a driver domain.  There's quite  
> a lot of interest in this for networking.

You can use veth for this. This works today.

> At any rate, I'd like to see performance results before we consider  
> trying to reuse virtio code.
> 
> Regards,
>
> Anthony Liguori
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On Thursday 13 August 2009, Michael S. Tsirkin wrote:
> On Wed, Aug 12, 2009 at 07:59:47PM +0200, Arnd Bergmann wrote:
> > The trick is to swap the virtqueues instead. virtio-net is actually
> > mostly symmetric in just the same way that the physical wires on a
> > twisted pair ethernet are symmetric (I like how that analogy fits).
> 
> You need to really squint hard for it to look symmetric.
> 
> For example, for RX, virtio allocates an skb, puts a descriptor on a
> ring and waits for host to fill it in. Host system can not do the same:
> guest does not have access to host memory.
> 
> You can do a copy in transport to hide this fact, but it will kill
> performance.

Yes, that is what I was suggesting all along. The actual copy operation
has to be done by the host transport, which is obviously different from
the guest transport that just calls the host using vring_kick().

Right now, the number of copy operations in your code is the same.
You are doing the copy a little bit later in skb_copy_datagram_iovec(),
which is indeed a very nice hack. Changing to a virtqueue based method
would imply that the host needs to add each skb_frag_t to its outbound
virtqueue, which then gets copied into the guests inbound virtqueue.

Unfortunately, this also implies that you could no longer simply use the
packet socket interface as you do currently, as I realized only now.
This obviously has a significant impact on your user space interface.

	Arnd <><
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On Wednesday 12 August 2009, Anthony Liguori wrote:
> At any rate, I'd like to see performance results before we consider 
> trying to reuse virtio code.

Yes, I agree. I'd also like to do more work on the macvlan extensions
to see if it works out without involving a socket. Passing the socket
into the vhost_net device is a nice feature of the current implementation
that we'd have to give up for something else (e.g. making the vhost
a real network interface that you can hook up to a bridge) if it were
to use virtio.

Unless I can come up with a solution that is clearly superior, I'm
taking back my objections on that part for now.

	Arnd <><
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On Thursday 13 August 2009, Arnd Bergmann wrote:
> Unfortunately, this also implies that you could no longer simply use the
> packet socket interface as you do currently, as I realized only now.
> This obviously has a significant impact on your user space interface.

Also, if we do the copy in the transport, it definitely means that we
can't get to zero-copy RX/TX from guest space any more. The current
vhost_net driver doesn't do that yet, but could be extended in the
same way that I'm hoping to do it for macvtap.

	Arnd <><
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On Thu, Aug 13, 2009 at 03:38:43PM +0200, Arnd Bergmann wrote:
> On Thursday 13 August 2009, Michael S. Tsirkin wrote:
> > On Wed, Aug 12, 2009 at 07:59:47PM +0200, Arnd Bergmann wrote:
> > > The trick is to swap the virtqueues instead. virtio-net is actually
> > > mostly symmetric in just the same way that the physical wires on a
> > > twisted pair ethernet are symmetric (I like how that analogy fits).
> > 
> > You need to really squint hard for it to look symmetric.
> > 
> > For example, for RX, virtio allocates an skb, puts a descriptor on a
> > ring and waits for host to fill it in. Host system can not do the same:
> > guest does not have access to host memory.
> > 
> > You can do a copy in transport to hide this fact, but it will kill
> > performance.
> 
> Yes, that is what I was suggesting all along. The actual copy operation
> has to be done by the host transport, which is obviously different from
> the guest transport that just calls the host using vring_kick().
> 
> Right now, the number of copy operations in your code is the same.
> You are doing the copy a little bit later in skb_copy_datagram_iovec(),
> which is indeed a very nice hack. Changing to a virtqueue based method
> would imply that the host needs to add each skb_frag_t to its outbound
> virtqueue, which then gets copied into the guests inbound virtqueue.

Which is a lot more code than just calling skb_copy_datagram_iovec.

> Unfortunately, this also implies that you could no longer simply use the
> packet socket interface as you do currently, as I realized only now.
> This obviously has a significant impact on your user space interface.
> 
> 	Arnd <><

And, it will remove our ability to implement zero copy
down the road (when raw sockets support it).

On Thu, Aug 13, 2009 at 03:48:35PM +0200, Arnd Bergmann wrote:
> On Thursday 13 August 2009, Arnd Bergmann wrote:
> > Unfortunately, this also implies that you could no longer simply use the
> > packet socket interface as you do currently, as I realized only now.
> > This obviously has a significant impact on your user space interface.
> 
> Also, if we do the copy in the transport, it definitely means that we
> can't get to zero-copy RX/TX from guest space any more. The current
> vhost_net driver doesn't do that yet, but could be extended in the
> same way that I'm hoping to do it for macvtap.
> 
> 	Arnd <><

The best way to do this IMO would be to add zero copy support to raw
sockets, vhost will then get it basically for free.

On Thursday 13 August 2009, Michael S. Tsirkin wrote:
> The best way to do this IMO would be to add zero copy support to raw
> sockets, vhost will then get it basically for free.

Yes, that would be nice. I wonder if that could lead to security
problems on TX though. I guess It will only bring significant performance
improvements if we leave the data writable in the user space or guest
during the operation. If the user finds the right timing, it could
modify the frame headers after they have been checked using netfilter,
or while the frames are being consumed in the kernel (e.g. an NFS
server running in a guest).

	Ardn <><
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On Thursday 13 August 2009, Michael S. Tsirkin wrote:
> > Right now, the number of copy operations in your code is the same.
> > You are doing the copy a little bit later in skb_copy_datagram_iovec(),
> > which is indeed a very nice hack. Changing to a virtqueue based method
> > would imply that the host needs to add each skb_frag_t to its outbound
> > virtqueue, which then gets copied into the guests inbound virtqueue.
> 
> Which is a lot more code than just calling skb_copy_datagram_iovec.

Well, I don't see this part as much of a problem, because the code
already exists in virtio_net. If we really wanted to go down that road,
just using virtio_net would solve the problem of frame handling
entirely, but create new problems elsewhere, as we have mentioned.

	Arnd <><
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On Thu, Aug 13, 2009 at 04:58:06PM +0200, Arnd Bergmann wrote:
> On Thursday 13 August 2009, Michael S. Tsirkin wrote:
> > > Right now, the number of copy operations in your code is the same.
> > > You are doing the copy a little bit later in skb_copy_datagram_iovec(),
> > > which is indeed a very nice hack. Changing to a virtqueue based method
> > > would imply that the host needs to add each skb_frag_t to its outbound
> > > virtqueue, which then gets copied into the guests inbound virtqueue.
> > 
> > Which is a lot more code than just calling skb_copy_datagram_iovec.
> 
> Well, I don't see this part as much of a problem, because the code
> already exists in virtio_net.

I am talking about the copying done in low level transport, here.

> If we really wanted to go down that road,
> just using virtio_net would solve the problem of frame handling
> entirely, but create new problems elsewhere, as we have mentioned.
> 
> 	Arnd <><
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On 08/13/2009 05:53 PM, Arnd Bergmann wrote:
> On Thursday 13 August 2009, Michael S. Tsirkin wrote:
>    
>> The best way to do this IMO would be to add zero copy support to raw
>> sockets, vhost will then get it basically for free.
>>      
>
> Yes, that would be nice. I wonder if that could lead to security
> problems on TX though. I guess It will only bring significant performance
> improvements if we leave the data writable in the user space or guest
> during the operation. If the user finds the right timing, it could
> modify the frame headers after they have been checked using netfilter,
> or while the frames are being consumed in the kernel (e.g. an NFS
> server running in a guest).
>    

IIRC when the kernel consumes data it linearizes the skb.  We just need 
to make sure all the zerocopy data is in the nonlinear part, and the 
kernel will copy if/when it needs to access packet data.

On Fri, 14 Aug 2009 12:23:46 am Arnd Bergmann wrote:
> On Thursday 13 August 2009, Michael S. Tsirkin wrote:
> > The best way to do this IMO would be to add zero copy support to raw
> > sockets, vhost will then get it basically for free.
> 
> Yes, that would be nice. I wonder if that could lead to security
> problems on TX though. I guess It will only bring significant performance
> improvements if we leave the data writable in the user space or guest
> during the operation. If the user finds the right timing, it could
> modify the frame headers after they have been checked using netfilter,
> or while the frames are being consumed in the kernel (e.g. an NFS
> server running in a guest).

For this reason, we always linearize parts of packets we're filtering.
ie. copy.

Cheers,
Rusty.
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