Re: [RFC v2 5/7] vhost: Add x-vhost-enable-shadow-vq qmp

[Eugenio Perez Martin <eperezma@redhat.com>]

On Wed, Feb 17, 2021 at 7:47 PM Eugenio Perez Martin
<eperezma@redhat.com> wrote:
>
> On Wed, Feb 17, 2021 at 4:26 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > On Tue, Feb 09, 2021 at 04:37:55PM +0100, Eugenio Pérez wrote:
> > > diff --git a/qapi/net.json b/qapi/net.json
> > > index c31748c87f..a1cdffb0f9 100644
> > > --- a/qapi/net.json
> > > +++ b/qapi/net.json
> > > @@ -77,6 +77,28 @@
> > >  ##
> > >  { 'command': 'netdev_del', 'data': {'id': 'str'} }
> > >
> > > +##
> > > +# @x-vhost-enable-shadow-vq:
> > > +#
> > > +# Use vhost shadow virtqueue.
> >
> > Is this command for testing only or do you expect it to be invoked by
> > libvirt in production? I think the shadow virtqueue can be an internal
> > QEMU feature that is hidden from management tools.
> >
>
> I think shadow virtqueue should kick in automatically when live
> migration is triggered and the vhost device does not have _F_LOG too.
>
> Maybe something like "prefer shadow vq to vhost logging" could be
> exposed, but it is not a thing we have to figure now.
>
> > > +#
> > > +# @name: the device name of the virtual network adapter
> > > +#
> > > +# @enable: true to use he alternate shadow VQ notification path
> > > +#
> > > +# Returns: Error if failure, or 'no error' for success
> > > +#
> > > +# Since: 6.0
> >
> > Is this a generic feature for any vhost or vDPA device? If yes, please
> > replace "virtual network adapter" in the doc comment.
> >
> > Does this only apply to vhost-net devices? If so, please put "vhost-net"
> > in the name since there are other non-net vhost devices.
>
> Right, thanks for the catch!
>

Moreover, the command should not be in net.json. However, I don't see
a generic virtio/vhost file to add the command.

> >
> > > +#
> > > +# Example:
> > > +#
> > > +# -> { "execute": "x-vhost-enable-shadow-vq", "arguments": {"enable": true} }
> >
> > Missing "name" field?
>
> Yes, I will fix the example in future revisions.
>
> Thanks!