Re: [Qemu-devel] [virtio-dev] [PATCH v3 0/7] Vhost-pci for inter-VM communication

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Dec 15, 2017 at 06:33:14PM +0800, Wei Wang wrote:
> On 12/14/2017 11:06 PM, Stefan Hajnoczi wrote:
> > On Wed, Dec 13, 2017 at 10:59:10PM +0200, Michael S. Tsirkin wrote:
> > > >   * VHOST_USER_SET_VRING_KICK
> > > > 
> > > >     Set up vring kick doorbell (unless bit 8 is set) before sending
> > > >     VHOST_USER_SET_VRING_KICK to the guest.
> > > But guest can't use it, now can it?
> > > 
> > > What guest needs is a mapping to interrupts.
> > ...
> > > >   * VHOST_USER_SET_VRING_CALL
> > > > 
> > > >     Set up the vring call doorbell (unless bit 8 is set) before sending
> > > >     VHOST_USER_SET_VRING_CALL to the guest.
> > > Same here. what guest needs is mapping from io to notifications,
> > > right?
> > The PCI device should contain a BAR with doorbell registers.  I don't
> > think a fancy mapping is necessary, instead the device spec should
> > define the BAR layout.
> > 
> > When the guest vhost-user slave receives this message it knows it can
> > now begin using the doorbell register.
> 
> Not really. A doorbell will cause an interrupt to be injected to the master
> device driver, which is not ready to work at that time. The slave driver
> isn't expected to use the doorbell until the master is ready by sending the
> last message VHOST_USER_SET_VHOST_PCI to link UP the slave device.
> 
> So I think passing the fd msg to guest doesn't have a value in terms of
> functionality.

The new VHOST_USER_SET_VHOST_PCI device message is something you defined
in this patch series.  This email sub-thread is about the vhost-user
protocol specification as it exist today.

The VHOST_USER_SET_VRING_CALL is the message that allows the guest to
begin using the doorbell when VHOST_USER_F_PROTOCOL_FEATURES wasn't
negotiated (i.e. the vring is enabled right away).

Stefan

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