On Fri, Jul 07, 2023 at 12:27:39PM +0200, Stefano Garzarella wrote:
> On Tue, Jul 04, 2023 at 04:02:42PM +0100, Alex Bennée wrote:
> > 
> > Stefano Garzarella <sgarzare@redhat.com> writes:
> > 
> > > On Tue, Jul 04, 2023 at 01:36:00PM +0100, Alex Bennée wrote:
> > > > diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> > > > index 5a070adbc1..85b1b1583a 100644
> > > > --- a/docs/interop/vhost-user.rst
> > > > +++ b/docs/interop/vhost-user.rst
> > > > @@ -275,6 +275,21 @@ Inflight description
> > > > 
> > > > :queue size: a 16-bit size of virtqueues
> > > > 
> > > > +Backend specifications
> > > > +^^^^^^^^^^^^^^^^^^^^^^
> > > > +
> > > > ++-----------+-------------+------------+------------+
> > > > +| device id | config size |   min_vqs  |   max_vqs  |
> > > > ++-----------+-------------+------------+------------+
> > > > +
> > > > +:device id: a 32-bit value holding the VirtIO device ID
> > > > +
> > > > +:config size: a 32-bit value holding the config size (see ``VHOST_USER_GET_CONFIG``)
> > > > +
> > > > +:min_vqs: a 32-bit value holding the minimum number of vqs supported
> > > 
> > > Why do we need the minimum?
> > 
> > We need to know the minimum number because some devices have fixed VQs
> > that must be present.
> 
> But does QEMU need to know this?
> 
> Or is it okay that the driver will then fail in the guest if there
> are not the right number of queues?

I don't understand why min_vqs is needed either. It's not the
front-end's job to ensure that the device will be used properly. A
spec-compliant driver will work with a spec-compliant device, so it's
not clear why the front-end needs this information.

Stefan