Re: [PATCH] virtio-net: clear guest_announce feature if no cvq backend

[Eugenio Perez Martin]

On Wed, Jan 25, 2023 at 4:20 PM David Edmondson
<david.edmondson@oracle.com> wrote:
>
> On Tuesday, 2023-01-24 at 17:11:59 +01, Eugenio Pérez wrote:
> > Since GUEST_ANNOUNCE is emulated the feature bit could be set without
> > backend support.  This happens in the vDPA case.
> >
> > However, backend vDPA parent may not have CVQ support.  This causes an
> > incoherent feature set, and the driver may refuse to start.  This
> > happens in virtio-net Linux driver.
>
> Could you now simplify the tests in virtio_net_announce() and
> virtio_net_post_load_device() to look only for the presence of
> GUEST_ANNOUNCE, given that you can now presume that it implies CTRL_VQ?
>

That's a good question. As far as I know qemu emits an error if only
GUEST_ANNOUNCE is given in a purely emulated device.

At this moment vhost-kernel and vhost-vdpa do not handle it, but
vhost-user do. Would it be beneficial to preserve previous behavior
and passthrough the features? I guess not, so I think we could
simplify those functions on top of this series.

> But anyway:
>
> Reviewed-by: David Edmondson <david.edmondson@oracle.com>
>

Thanks for the review!

> > This may be solved differently in the future.  Qemu is able to emulate a
> > CVQ just for guest_announce purposes, helping guest to notify the new
> > location with vDPA devices that does not support it.  However, this is
> > left as a TODO as it is way more complex to backport.
> >
> > Tested with vdpa_net_sim, toggling manually VIRTIO_NET_F_CTRL_VQ in the
> > driver and migrating it with x-svq=on.
> >
> > Fixes: 980003debddd ("vdpa: do not handle VIRTIO_NET_F_GUEST_ANNOUNCE in 
> > vhost-vdpa")
> > Reported-by: Dawar, Gautam <gautam.dawar@amd.com>
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >  hw/net/virtio-net.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> >
> > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > index 3ae909041a..09d5c7a664 100644
> > --- a/hw/net/virtio-net.c
> > +++ b/hw/net/virtio-net.c
> > @@ -820,6 +820,21 @@ static uint64_t virtio_net_get_features(VirtIODevice 
> > *vdev, uint64_t features,
> >          features |= (1ULL << VIRTIO_NET_F_MTU);
> >      }
> >
> > +    /*
> > +     * Since GUEST_ANNOUNCE is emulated the feature bit could be set 
> > without
> > +     * enabled. This happens in the vDPA case.
> > +     *
> > +     * Make sure the feature set is not incoherent, as the driver could 
> > refuse
> > +     * to start.
> > +     *
> > +     * TODO: QEMU is able to emulate a CVQ just for guest_announce 
> > purposes,
> > +     * helping guest to notify the new location with vDPA devices that 
> > does not
> > +     * support it.
> > +     */
> > +    if (!virtio_has_feature(vdev->backend_features, VIRTIO_NET_F_CTRL_VQ)) 
> > {
> > +        virtio_clear_feature(&features, VIRTIO_NET_F_GUEST_ANNOUNCE);
> > +    }
> > +
> >      return features;
> >  }
> --
> Why stay in college? Why go to night school?
>