[net-next,12/14] vsock/vmci: register vmci_transport only when VMCI guest/host are active
diff mbox series

Message ID 20191023095554.11340-13-sgarzare@redhat.com
State Changes Requested
Delegated to: David Miller
Headers show
Series
  • vsock: add multi-transports support
Related show

Commit Message

Stefano Garzarella Oct. 23, 2019, 9:55 a.m. UTC
To allow other transports to be loaded with vmci_transport,
we register the vmci_transport as G2H or H2G only when a VMCI guest
or host is active.

To do that, this patch adds a callback registered in the vmci driver
that will be called when a new host or guest become active.
This callback will register the vmci_transport in the VSOCK core.
If the transport is already registered, we ignore the error coming
from vsock_core_register().

Cc: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
 drivers/misc/vmw_vmci/vmci_driver.c | 50 +++++++++++++++++++++++++++++
 drivers/misc/vmw_vmci/vmci_driver.h |  2 ++
 drivers/misc/vmw_vmci/vmci_guest.c  |  2 ++
 drivers/misc/vmw_vmci/vmci_host.c   |  7 ++++
 include/linux/vmw_vmci_api.h        |  2 ++
 net/vmw_vsock/vmci_transport.c      | 29 +++++++++++------
 6 files changed, 82 insertions(+), 10 deletions(-)

Comments

Stefan Hajnoczi Oct. 27, 2019, 8:17 a.m. UTC | #1
On Wed, Oct 23, 2019 at 11:55:52AM +0200, Stefano Garzarella wrote:
> +static int __init vmci_transport_init(void)
> +{
> +	int features = VSOCK_TRANSPORT_F_DGRAM;

Where is this variable used?
Stefano Garzarella Oct. 29, 2019, 4:35 p.m. UTC | #2
On Sun, Oct 27, 2019 at 09:17:52AM +0100, Stefan Hajnoczi wrote:
> On Wed, Oct 23, 2019 at 11:55:52AM +0200, Stefano Garzarella wrote:
> > +static int __init vmci_transport_init(void)
> > +{
> > +	int features = VSOCK_TRANSPORT_F_DGRAM;
> 
> Where is this variable used?

It is introduced in the previous patch "vsock: add multi-transports support",
and it is used in the vsock_core_register(), but since now the
vmci_transport_init() registers the vmci_transport only with DGRAM
feature, I can remove this variable and I can use directly the
VSOCK_TRANSPORT_F_DGRAM.

I'll fix in the v3.

Thanks,
Stefano
Stefano Garzarella Nov. 4, 2019, 10:10 a.m. UTC | #3
Hi Jorgen,
I'm preparing the v2, but first, if you have time, I'd like to have
a comment from you on this patch that modifies a bit vmci.

Thank you very much,
Stefano

On Wed, Oct 23, 2019 at 11:55:52AM +0200, Stefano Garzarella wrote:
> To allow other transports to be loaded with vmci_transport,
> we register the vmci_transport as G2H or H2G only when a VMCI guest
> or host is active.
> 
> To do that, this patch adds a callback registered in the vmci driver
> that will be called when a new host or guest become active.
> This callback will register the vmci_transport in the VSOCK core.
> If the transport is already registered, we ignore the error coming
> from vsock_core_register().
> 
> Cc: Jorgen Hansen <jhansen@vmware.com>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>  drivers/misc/vmw_vmci/vmci_driver.c | 50 +++++++++++++++++++++++++++++
>  drivers/misc/vmw_vmci/vmci_driver.h |  2 ++
>  drivers/misc/vmw_vmci/vmci_guest.c  |  2 ++
>  drivers/misc/vmw_vmci/vmci_host.c   |  7 ++++
>  include/linux/vmw_vmci_api.h        |  2 ++
>  net/vmw_vsock/vmci_transport.c      | 29 +++++++++++------
>  6 files changed, 82 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/misc/vmw_vmci/vmci_driver.c b/drivers/misc/vmw_vmci/vmci_driver.c
> index 819e35995d32..195afbd7edc1 100644
> --- a/drivers/misc/vmw_vmci/vmci_driver.c
> +++ b/drivers/misc/vmw_vmci/vmci_driver.c
> @@ -28,6 +28,9 @@ MODULE_PARM_DESC(disable_guest,
>  static bool vmci_guest_personality_initialized;
>  static bool vmci_host_personality_initialized;
>  
> +static DEFINE_MUTEX(vmci_vsock_mutex); /* protects vmci_vsock_transport_cb */
> +static vmci_vsock_cb vmci_vsock_transport_cb;
> +
>  /*
>   * vmci_get_context_id() - Gets the current context ID.
>   *
> @@ -45,6 +48,53 @@ u32 vmci_get_context_id(void)
>  }
>  EXPORT_SYMBOL_GPL(vmci_get_context_id);
>  
> +/*
> + * vmci_register_vsock_callback() - Register the VSOCK vmci_transport callback.
> + *
> + * The callback will be called every time a new host or guest become active,
> + * or if they are already active when this function is called.
> + * To unregister the callback, call this function with NULL parameter.
> + *
> + * Returns 0 on success. -EBUSY if a callback is already registered.
> + */
> +int vmci_register_vsock_callback(vmci_vsock_cb callback)
> +{
> +	int err = 0;
> +
> +	mutex_lock(&vmci_vsock_mutex);
> +
> +	if (vmci_vsock_transport_cb && callback) {
> +		err = -EBUSY;
> +		goto out;
> +	}
> +
> +	vmci_vsock_transport_cb = callback;
> +
> +	if (!vmci_vsock_transport_cb)
> +		goto out;
> +
> +	if (vmci_guest_code_active())
> +		vmci_vsock_transport_cb(false);
> +
> +	if (vmci_host_users() > 0)
> +		vmci_vsock_transport_cb(true);
> +
> +out:
> +	mutex_unlock(&vmci_vsock_mutex);
> +	return err;
> +}
> +EXPORT_SYMBOL_GPL(vmci_register_vsock_callback);
> +
> +void vmci_call_vsock_callback(bool is_host)
> +{
> +	mutex_lock(&vmci_vsock_mutex);
> +
> +	if (vmci_vsock_transport_cb)
> +		vmci_vsock_transport_cb(is_host);
> +
> +	mutex_unlock(&vmci_vsock_mutex);
> +}
> +
>  static int __init vmci_drv_init(void)
>  {
>  	int vmci_err;
> diff --git a/drivers/misc/vmw_vmci/vmci_driver.h b/drivers/misc/vmw_vmci/vmci_driver.h
> index aab81b67670c..990682480bf6 100644
> --- a/drivers/misc/vmw_vmci/vmci_driver.h
> +++ b/drivers/misc/vmw_vmci/vmci_driver.h
> @@ -36,10 +36,12 @@ extern struct pci_dev *vmci_pdev;
>  
>  u32 vmci_get_context_id(void);
>  int vmci_send_datagram(struct vmci_datagram *dg);
> +void vmci_call_vsock_callback(bool is_host);
>  
>  int vmci_host_init(void);
>  void vmci_host_exit(void);
>  bool vmci_host_code_active(void);
> +int vmci_host_users(void);
>  
>  int vmci_guest_init(void);
>  void vmci_guest_exit(void);
> diff --git a/drivers/misc/vmw_vmci/vmci_guest.c b/drivers/misc/vmw_vmci/vmci_guest.c
> index 7a84a48c75da..cc8eeb361fcd 100644
> --- a/drivers/misc/vmw_vmci/vmci_guest.c
> +++ b/drivers/misc/vmw_vmci/vmci_guest.c
> @@ -637,6 +637,8 @@ static int vmci_guest_probe_device(struct pci_dev *pdev,
>  		  vmci_dev->iobase + VMCI_CONTROL_ADDR);
>  
>  	pci_set_drvdata(pdev, vmci_dev);
> +
> +	vmci_call_vsock_callback(false);
>  	return 0;
>  
>  err_free_irq:
> diff --git a/drivers/misc/vmw_vmci/vmci_host.c b/drivers/misc/vmw_vmci/vmci_host.c
> index 833e2bd248a5..ff3c396146ff 100644
> --- a/drivers/misc/vmw_vmci/vmci_host.c
> +++ b/drivers/misc/vmw_vmci/vmci_host.c
> @@ -108,6 +108,11 @@ bool vmci_host_code_active(void)
>  	     atomic_read(&vmci_host_active_users) > 0);
>  }
>  
> +int vmci_host_users(void)
> +{
> +	return atomic_read(&vmci_host_active_users);
> +}
> +
>  /*
>   * Called on open of /dev/vmci.
>   */
> @@ -338,6 +343,8 @@ static int vmci_host_do_init_context(struct vmci_host_dev *vmci_host_dev,
>  	vmci_host_dev->ct_type = VMCIOBJ_CONTEXT;
>  	atomic_inc(&vmci_host_active_users);
>  
> +	vmci_call_vsock_callback(true);
> +
>  	retval = 0;
>  
>  out:
> diff --git a/include/linux/vmw_vmci_api.h b/include/linux/vmw_vmci_api.h
> index acd9fafe4fc6..f28907345c80 100644
> --- a/include/linux/vmw_vmci_api.h
> +++ b/include/linux/vmw_vmci_api.h
> @@ -19,6 +19,7 @@
>  struct msghdr;
>  typedef void (vmci_device_shutdown_fn) (void *device_registration,
>  					void *user_data);
> +typedef void (*vmci_vsock_cb) (bool is_host);
>  
>  int vmci_datagram_create_handle(u32 resource_id, u32 flags,
>  				vmci_datagram_recv_cb recv_cb,
> @@ -37,6 +38,7 @@ int vmci_doorbell_destroy(struct vmci_handle handle);
>  int vmci_doorbell_notify(struct vmci_handle handle, u32 priv_flags);
>  u32 vmci_get_context_id(void);
>  bool vmci_is_context_owner(u32 context_id, kuid_t uid);
> +int vmci_register_vsock_callback(vmci_vsock_cb callback);
>  
>  int vmci_event_subscribe(u32 event,
>  			 vmci_event_cb callback, void *callback_data,
> diff --git a/net/vmw_vsock/vmci_transport.c b/net/vmw_vsock/vmci_transport.c
> index 2eb3f16d53e7..04437f822d82 100644
> --- a/net/vmw_vsock/vmci_transport.c
> +++ b/net/vmw_vsock/vmci_transport.c
> @@ -2053,19 +2053,22 @@ static bool vmci_check_transport(struct vsock_sock *vsk)
>  	return vsk->transport == &vmci_transport;
>  }
>  
> -static int __init vmci_transport_init(void)
> +void vmci_vsock_transport_cb(bool is_host)
>  {
> -	int features = VSOCK_TRANSPORT_F_DGRAM | VSOCK_TRANSPORT_F_H2G;
> -	int cid;
> -	int err;
> +	int features;
>  
> -	cid = vmci_get_context_id();
> +	if (is_host)
> +		features = VSOCK_TRANSPORT_F_H2G;
> +	else
> +		features = VSOCK_TRANSPORT_F_G2H;
>  
> -	if (cid == VMCI_INVALID_ID)
> -		return -EINVAL;
> +	vsock_core_register(&vmci_transport, features);
> +}
>  
> -	if (cid != VMCI_HOST_CONTEXT_ID)
> -		features |= VSOCK_TRANSPORT_F_G2H;
> +static int __init vmci_transport_init(void)
> +{
> +	int features = VSOCK_TRANSPORT_F_DGRAM;
> +	int err;
>  
>  	/* Create the datagram handle that we will use to send and receive all
>  	 * VSocket control messages for this context.
> @@ -2079,7 +2082,6 @@ static int __init vmci_transport_init(void)
>  		pr_err("Unable to create datagram handle. (%d)\n", err);
>  		return vmci_transport_error_to_vsock_error(err);
>  	}
> -
>  	err = vmci_event_subscribe(VMCI_EVENT_QP_RESUMED,
>  				   vmci_transport_qp_resumed_cb,
>  				   NULL, &vmci_transport_qp_resumed_sub_id);
> @@ -2094,8 +2096,14 @@ static int __init vmci_transport_init(void)
>  	if (err < 0)
>  		goto err_unsubscribe;
>  
> +	err = vmci_register_vsock_callback(vmci_vsock_transport_cb);
> +	if (err < 0)
> +		goto err_unregister;
> +
>  	return 0;
>  
> +err_unregister:
> +	vsock_core_unregister(&vmci_transport);
>  err_unsubscribe:
>  	vmci_event_unsubscribe(vmci_transport_qp_resumed_sub_id);
>  err_destroy_stream_handle:
> @@ -2121,6 +2129,7 @@ static void __exit vmci_transport_exit(void)
>  		vmci_transport_qp_resumed_sub_id = VMCI_INVALID_ID;
>  	}
>  
> +	vmci_register_vsock_callback(NULL);
>  	vsock_core_unregister(&vmci_transport);
>  }
>  module_exit(vmci_transport_exit);
> -- 
> 2.21.0
> 

--
Jorgen Hansen Nov. 11, 2019, 4:27 p.m. UTC | #4
> From: Stefano Garzarella [mailto:sgarzare@redhat.com]
> Sent: Wednesday, October 23, 2019 11:56 AM
> 
> To allow other transports to be loaded with vmci_transport,
> we register the vmci_transport as G2H or H2G only when a VMCI guest
> or host is active.
> 
> To do that, this patch adds a callback registered in the vmci driver
> that will be called when a new host or guest become active.
> This callback will register the vmci_transport in the VSOCK core.
> If the transport is already registered, we ignore the error coming
> from vsock_core_register().

So today this is mainly an issue for the VMCI vsock transport, because
VMCI autoloads with vsock (and with this solution it can continue to
do that, so none of our old products break due to changed behavior,
which is great). Shouldn't vhost behave similar, so that any module
that registers a h2g transport only does so if it is in active use?


> --- a/drivers/misc/vmw_vmci/vmci_host.c
> +++ b/drivers/misc/vmw_vmci/vmci_host.c
> @@ -108,6 +108,11 @@ bool vmci_host_code_active(void)
>  	     atomic_read(&vmci_host_active_users) > 0);
>  }
> 
> +int vmci_host_users(void)
> +{
> +	return atomic_read(&vmci_host_active_users);
> +}
> +
>  /*
>   * Called on open of /dev/vmci.
>   */
> @@ -338,6 +343,8 @@ static int vmci_host_do_init_context(struct
> vmci_host_dev *vmci_host_dev,
>  	vmci_host_dev->ct_type = VMCIOBJ_CONTEXT;
>  	atomic_inc(&vmci_host_active_users);
> 
> +	vmci_call_vsock_callback(true);
> +

Since we don't unregister the transport if user count drops back to 0, we could
just call this the first time, a VM is powered on after the module is loaded.

>  	retval = 0;
> 
>  out:
Stefano Garzarella Nov. 11, 2019, 5:30 p.m. UTC | #5
On Mon, Nov 11, 2019 at 04:27:28PM +0000, Jorgen Hansen wrote:
> > From: Stefano Garzarella [mailto:sgarzare@redhat.com]
> > Sent: Wednesday, October 23, 2019 11:56 AM
> > 
> > To allow other transports to be loaded with vmci_transport,
> > we register the vmci_transport as G2H or H2G only when a VMCI guest
> > or host is active.
> > 
> > To do that, this patch adds a callback registered in the vmci driver
> > that will be called when a new host or guest become active.
> > This callback will register the vmci_transport in the VSOCK core.
> > If the transport is already registered, we ignore the error coming
> > from vsock_core_register().
> 
> So today this is mainly an issue for the VMCI vsock transport, because
> VMCI autoloads with vsock (and with this solution it can continue to
> do that, so none of our old products break due to changed behavior,
> which is great).

I tried to not break anything :-)

>                  Shouldn't vhost behave similar, so that any module
> that registers a h2g transport only does so if it is in active use?
> 

The vhost-vsock module will load when the first hypervisor open
/dev/vhost-vsock, so in theory, when there's at least one active user.

> 
> > --- a/drivers/misc/vmw_vmci/vmci_host.c
> > +++ b/drivers/misc/vmw_vmci/vmci_host.c
> > @@ -108,6 +108,11 @@ bool vmci_host_code_active(void)
> >  	     atomic_read(&vmci_host_active_users) > 0);
> >  }
> > 
> > +int vmci_host_users(void)
> > +{
> > +	return atomic_read(&vmci_host_active_users);
> > +}
> > +
> >  /*
> >   * Called on open of /dev/vmci.
> >   */
> > @@ -338,6 +343,8 @@ static int vmci_host_do_init_context(struct
> > vmci_host_dev *vmci_host_dev,
> >  	vmci_host_dev->ct_type = VMCIOBJ_CONTEXT;
> >  	atomic_inc(&vmci_host_active_users);
> > 
> > +	vmci_call_vsock_callback(true);
> > +
> 
> Since we don't unregister the transport if user count drops back to 0, we could
> just call this the first time, a VM is powered on after the module is loaded.

Yes, make sense. can I use the 'vmci_host_active_users' or is better to
add a new 'vmci_host_vsock_loaded'?

My doubt is that vmci_host_active_users can return to 0, so when it returns
to 1, we call vmci_call_vsock_callback() again.

Thanks,
Stefano
Jorgen Hansen Nov. 12, 2019, 10:03 a.m. UTC | #6
> From: Stefano Garzarella [mailto:sgarzare@redhat.com]
> Sent: Monday, November 11, 2019 6:31 PM
> On Mon, Nov 11, 2019 at 04:27:28PM +0000, Jorgen Hansen wrote:
> > > From: Stefano Garzarella [mailto:sgarzare@redhat.com]
> > > Sent: Wednesday, October 23, 2019 11:56 AM
> > >
> > > To allow other transports to be loaded with vmci_transport,
> > > we register the vmci_transport as G2H or H2G only when a VMCI guest
> > > or host is active.
> > >
> > > To do that, this patch adds a callback registered in the vmci driver
> > > that will be called when a new host or guest become active.
> > > This callback will register the vmci_transport in the VSOCK core.
> > > If the transport is already registered, we ignore the error coming
> > > from vsock_core_register().
> >
> > So today this is mainly an issue for the VMCI vsock transport, because
> > VMCI autoloads with vsock (and with this solution it can continue to
> > do that, so none of our old products break due to changed behavior,
> > which is great).
> 
> I tried to not break anything :-)
> 
> >                  Shouldn't vhost behave similar, so that any module
> > that registers a h2g transport only does so if it is in active use?
> >
> 
> The vhost-vsock module will load when the first hypervisor open
> /dev/vhost-vsock, so in theory, when there's at least one active user.

Ok, sounds good then. 

> 
> >
> > > --- a/drivers/misc/vmw_vmci/vmci_host.c
> > > +++ b/drivers/misc/vmw_vmci/vmci_host.c
> > > @@ -108,6 +108,11 @@ bool vmci_host_code_active(void)
> > >  	     atomic_read(&vmci_host_active_users) > 0);
> > >  }
> > >
> > > +int vmci_host_users(void)
> > > +{
> > > +	return atomic_read(&vmci_host_active_users);
> > > +}
> > > +
> > >  /*
> > >   * Called on open of /dev/vmci.
> > >   */
> > > @@ -338,6 +343,8 @@ static int vmci_host_do_init_context(struct
> > > vmci_host_dev *vmci_host_dev,
> > >  	vmci_host_dev->ct_type = VMCIOBJ_CONTEXT;
> > >  	atomic_inc(&vmci_host_active_users);
> > >
> > > +	vmci_call_vsock_callback(true);
> > > +
> >
> > Since we don't unregister the transport if user count drops back to 0, we
> could
> > just call this the first time, a VM is powered on after the module is loaded.
> 
> Yes, make sense. can I use the 'vmci_host_active_users' or is better to
> add a new 'vmci_host_vsock_loaded'?
> 
> My doubt is that vmci_host_active_users can return to 0, so when it returns
> to 1, we call vmci_call_vsock_callback() again.

vmci_host_active_users can drop to 0 and then increase again, so having a flag
indicating whether the callback has been invoked would ensure that it is only
called once.

Thanks,
Jorgen
Stefano Garzarella Nov. 12, 2019, 10:42 a.m. UTC | #7
On Tue, Nov 12, 2019 at 10:03:54AM +0000, Jorgen Hansen wrote:
> > From: Stefano Garzarella [mailto:sgarzare@redhat.com]
> > Sent: Monday, November 11, 2019 6:31 PM
> > On Mon, Nov 11, 2019 at 04:27:28PM +0000, Jorgen Hansen wrote:
> > > > From: Stefano Garzarella [mailto:sgarzare@redhat.com]
> > > > Sent: Wednesday, October 23, 2019 11:56 AM
> > > >
> > > > To allow other transports to be loaded with vmci_transport,
> > > > we register the vmci_transport as G2H or H2G only when a VMCI guest
> > > > or host is active.
> > > >
> > > > To do that, this patch adds a callback registered in the vmci driver
> > > > that will be called when a new host or guest become active.
> > > > This callback will register the vmci_transport in the VSOCK core.
> > > > If the transport is already registered, we ignore the error coming
> > > > from vsock_core_register().
> > >
> > > So today this is mainly an issue for the VMCI vsock transport, because
> > > VMCI autoloads with vsock (and with this solution it can continue to
> > > do that, so none of our old products break due to changed behavior,
> > > which is great).
> > 
> > I tried to not break anything :-)
> > 
> > >                  Shouldn't vhost behave similar, so that any module
> > > that registers a h2g transport only does so if it is in active use?
> > >
> > 
> > The vhost-vsock module will load when the first hypervisor open
> > /dev/vhost-vsock, so in theory, when there's at least one active user.
> 
> Ok, sounds good then. 
> 
> > 
> > >
> > > > --- a/drivers/misc/vmw_vmci/vmci_host.c
> > > > +++ b/drivers/misc/vmw_vmci/vmci_host.c
> > > > @@ -108,6 +108,11 @@ bool vmci_host_code_active(void)
> > > >  	     atomic_read(&vmci_host_active_users) > 0);
> > > >  }
> > > >
> > > > +int vmci_host_users(void)
> > > > +{
> > > > +	return atomic_read(&vmci_host_active_users);
> > > > +}
> > > > +
> > > >  /*
> > > >   * Called on open of /dev/vmci.
> > > >   */
> > > > @@ -338,6 +343,8 @@ static int vmci_host_do_init_context(struct
> > > > vmci_host_dev *vmci_host_dev,
> > > >  	vmci_host_dev->ct_type = VMCIOBJ_CONTEXT;
> > > >  	atomic_inc(&vmci_host_active_users);
> > > >
> > > > +	vmci_call_vsock_callback(true);
> > > > +
> > >
> > > Since we don't unregister the transport if user count drops back to 0, we
> > could
> > > just call this the first time, a VM is powered on after the module is loaded.
> > 
> > Yes, make sense. can I use the 'vmci_host_active_users' or is better to
> > add a new 'vmci_host_vsock_loaded'?
> > 
> > My doubt is that vmci_host_active_users can return to 0, so when it returns
> > to 1, we call vmci_call_vsock_callback() again.
> 
> vmci_host_active_users can drop to 0 and then increase again, so having a flag
> indicating whether the callback has been invoked would ensure that it is only
> called once.

I agree, I will use a dedicated flag, maybe in the
vmci_call_vsock_callback(), since it can be called or during the
vmci_host_do_init_context() or when the callback is registered.

Thanks,
Stefano

Patch
diff mbox series

diff --git a/drivers/misc/vmw_vmci/vmci_driver.c b/drivers/misc/vmw_vmci/vmci_driver.c
index 819e35995d32..195afbd7edc1 100644
--- a/drivers/misc/vmw_vmci/vmci_driver.c
+++ b/drivers/misc/vmw_vmci/vmci_driver.c
@@ -28,6 +28,9 @@  MODULE_PARM_DESC(disable_guest,
 static bool vmci_guest_personality_initialized;
 static bool vmci_host_personality_initialized;
 
+static DEFINE_MUTEX(vmci_vsock_mutex); /* protects vmci_vsock_transport_cb */
+static vmci_vsock_cb vmci_vsock_transport_cb;
+
 /*
  * vmci_get_context_id() - Gets the current context ID.
  *
@@ -45,6 +48,53 @@  u32 vmci_get_context_id(void)
 }
 EXPORT_SYMBOL_GPL(vmci_get_context_id);
 
+/*
+ * vmci_register_vsock_callback() - Register the VSOCK vmci_transport callback.
+ *
+ * The callback will be called every time a new host or guest become active,
+ * or if they are already active when this function is called.
+ * To unregister the callback, call this function with NULL parameter.
+ *
+ * Returns 0 on success. -EBUSY if a callback is already registered.
+ */
+int vmci_register_vsock_callback(vmci_vsock_cb callback)
+{
+	int err = 0;
+
+	mutex_lock(&vmci_vsock_mutex);
+
+	if (vmci_vsock_transport_cb && callback) {
+		err = -EBUSY;
+		goto out;
+	}
+
+	vmci_vsock_transport_cb = callback;
+
+	if (!vmci_vsock_transport_cb)
+		goto out;
+
+	if (vmci_guest_code_active())
+		vmci_vsock_transport_cb(false);
+
+	if (vmci_host_users() > 0)
+		vmci_vsock_transport_cb(true);
+
+out:
+	mutex_unlock(&vmci_vsock_mutex);
+	return err;
+}
+EXPORT_SYMBOL_GPL(vmci_register_vsock_callback);
+
+void vmci_call_vsock_callback(bool is_host)
+{
+	mutex_lock(&vmci_vsock_mutex);
+
+	if (vmci_vsock_transport_cb)
+		vmci_vsock_transport_cb(is_host);
+
+	mutex_unlock(&vmci_vsock_mutex);
+}
+
 static int __init vmci_drv_init(void)
 {
 	int vmci_err;
diff --git a/drivers/misc/vmw_vmci/vmci_driver.h b/drivers/misc/vmw_vmci/vmci_driver.h
index aab81b67670c..990682480bf6 100644
--- a/drivers/misc/vmw_vmci/vmci_driver.h
+++ b/drivers/misc/vmw_vmci/vmci_driver.h
@@ -36,10 +36,12 @@  extern struct pci_dev *vmci_pdev;
 
 u32 vmci_get_context_id(void);
 int vmci_send_datagram(struct vmci_datagram *dg);
+void vmci_call_vsock_callback(bool is_host);
 
 int vmci_host_init(void);
 void vmci_host_exit(void);
 bool vmci_host_code_active(void);
+int vmci_host_users(void);
 
 int vmci_guest_init(void);
 void vmci_guest_exit(void);
diff --git a/drivers/misc/vmw_vmci/vmci_guest.c b/drivers/misc/vmw_vmci/vmci_guest.c
index 7a84a48c75da..cc8eeb361fcd 100644
--- a/drivers/misc/vmw_vmci/vmci_guest.c
+++ b/drivers/misc/vmw_vmci/vmci_guest.c
@@ -637,6 +637,8 @@  static int vmci_guest_probe_device(struct pci_dev *pdev,
 		  vmci_dev->iobase + VMCI_CONTROL_ADDR);
 
 	pci_set_drvdata(pdev, vmci_dev);
+
+	vmci_call_vsock_callback(false);
 	return 0;
 
 err_free_irq:
diff --git a/drivers/misc/vmw_vmci/vmci_host.c b/drivers/misc/vmw_vmci/vmci_host.c
index 833e2bd248a5..ff3c396146ff 100644
--- a/drivers/misc/vmw_vmci/vmci_host.c
+++ b/drivers/misc/vmw_vmci/vmci_host.c
@@ -108,6 +108,11 @@  bool vmci_host_code_active(void)
 	     atomic_read(&vmci_host_active_users) > 0);
 }
 
+int vmci_host_users(void)
+{
+	return atomic_read(&vmci_host_active_users);
+}
+
 /*
  * Called on open of /dev/vmci.
  */
@@ -338,6 +343,8 @@  static int vmci_host_do_init_context(struct vmci_host_dev *vmci_host_dev,
 	vmci_host_dev->ct_type = VMCIOBJ_CONTEXT;
 	atomic_inc(&vmci_host_active_users);
 
+	vmci_call_vsock_callback(true);
+
 	retval = 0;
 
 out:
diff --git a/include/linux/vmw_vmci_api.h b/include/linux/vmw_vmci_api.h
index acd9fafe4fc6..f28907345c80 100644
--- a/include/linux/vmw_vmci_api.h
+++ b/include/linux/vmw_vmci_api.h
@@ -19,6 +19,7 @@ 
 struct msghdr;
 typedef void (vmci_device_shutdown_fn) (void *device_registration,
 					void *user_data);
+typedef void (*vmci_vsock_cb) (bool is_host);
 
 int vmci_datagram_create_handle(u32 resource_id, u32 flags,
 				vmci_datagram_recv_cb recv_cb,
@@ -37,6 +38,7 @@  int vmci_doorbell_destroy(struct vmci_handle handle);
 int vmci_doorbell_notify(struct vmci_handle handle, u32 priv_flags);
 u32 vmci_get_context_id(void);
 bool vmci_is_context_owner(u32 context_id, kuid_t uid);
+int vmci_register_vsock_callback(vmci_vsock_cb callback);
 
 int vmci_event_subscribe(u32 event,
 			 vmci_event_cb callback, void *callback_data,
diff --git a/net/vmw_vsock/vmci_transport.c b/net/vmw_vsock/vmci_transport.c
index 2eb3f16d53e7..04437f822d82 100644
--- a/net/vmw_vsock/vmci_transport.c
+++ b/net/vmw_vsock/vmci_transport.c
@@ -2053,19 +2053,22 @@  static bool vmci_check_transport(struct vsock_sock *vsk)
 	return vsk->transport == &vmci_transport;
 }
 
-static int __init vmci_transport_init(void)
+void vmci_vsock_transport_cb(bool is_host)
 {
-	int features = VSOCK_TRANSPORT_F_DGRAM | VSOCK_TRANSPORT_F_H2G;
-	int cid;
-	int err;
+	int features;
 
-	cid = vmci_get_context_id();
+	if (is_host)
+		features = VSOCK_TRANSPORT_F_H2G;
+	else
+		features = VSOCK_TRANSPORT_F_G2H;
 
-	if (cid == VMCI_INVALID_ID)
-		return -EINVAL;
+	vsock_core_register(&vmci_transport, features);
+}
 
-	if (cid != VMCI_HOST_CONTEXT_ID)
-		features |= VSOCK_TRANSPORT_F_G2H;
+static int __init vmci_transport_init(void)
+{
+	int features = VSOCK_TRANSPORT_F_DGRAM;
+	int err;
 
 	/* Create the datagram handle that we will use to send and receive all
 	 * VSocket control messages for this context.
@@ -2079,7 +2082,6 @@  static int __init vmci_transport_init(void)
 		pr_err("Unable to create datagram handle. (%d)\n", err);
 		return vmci_transport_error_to_vsock_error(err);
 	}
-
 	err = vmci_event_subscribe(VMCI_EVENT_QP_RESUMED,
 				   vmci_transport_qp_resumed_cb,
 				   NULL, &vmci_transport_qp_resumed_sub_id);
@@ -2094,8 +2096,14 @@  static int __init vmci_transport_init(void)
 	if (err < 0)
 		goto err_unsubscribe;
 
+	err = vmci_register_vsock_callback(vmci_vsock_transport_cb);
+	if (err < 0)
+		goto err_unregister;
+
 	return 0;
 
+err_unregister:
+	vsock_core_unregister(&vmci_transport);
 err_unsubscribe:
 	vmci_event_unsubscribe(vmci_transport_qp_resumed_sub_id);
 err_destroy_stream_handle:
@@ -2121,6 +2129,7 @@  static void __exit vmci_transport_exit(void)
 		vmci_transport_qp_resumed_sub_id = VMCI_INVALID_ID;
 	}
 
+	vmci_register_vsock_callback(NULL);
 	vsock_core_unregister(&vmci_transport);
 }
 module_exit(vmci_transport_exit);