Re: [Qemu-devel] [RFC] vhost-user: don't ignore CTRL_VLAN feature

[Michael S. Tsirkin]

On Wed, Jun 12, 2019 at 10:11:57AM +0800, Tiwei Bie wrote:
> On Tue, Jun 11, 2019 at 10:10:14AM -0400, Michael S. Tsirkin wrote:
> > On Tue, Jun 11, 2019 at 02:51:37PM +0800, Tiwei Bie wrote:
> > > The VIRTIO_NET_F_CTRL_VLAN feature requires the support of
> > > vhost-user backend. But it will be advertised to guest driver
> > > as long as it's enabled by users in QEMU, while it's not
> > > supported by vhost-user backend. This patch fixes this issue.
> > 
> > Fixes by making guest refuse to send vlan tags?
> 
> Fixes by not advertising this feature bit to guest driver
> when it's not supported, and guest won't expect the device
> to do vlan filtering then.
> 
> > I agree it seems cleaner, but which guests does this actually help?
> > 
> > > Fixes: 72018d1e1917 ("vhost-user: ignore qemu-only features")
> > > Cc: address@hidden
> > > 
> > > Signed-off-by: Tiwei Bie <address@hidden>
> > 
> > A change like that will break migration compatibility, will it not?
> 
> Yeah, that's a problem...
> 
> > Maybe we need to tie it to a machine version somehow...
> > 
> > 
> > > ---
> > > It's not clear in the spec that, whether vlan filtering is
> > > also best-effort:
> > > https://github.com/oasis-tcs/virtio-spec/blob/37057052e7/content.tex#L3372
> > 
> > So what breaks if we declare it best effort for now?
> > And does it really help if we report that vlan filtering
> > is not supported to guests?
> 
> If it's best effort, then it won't violate the spec to
> advertise this feature when it's not supported in backends.
> 

OK so it's up to you. Want to propose a spec patch?
We already say mac filter is best effort ...

> > 
> > 
> > 
> > >  hw/net/vhost_net.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> > > index a6b719035c..1444fc9230 100644
> > > --- a/hw/net/vhost_net.c
> > > +++ b/hw/net/vhost_net.c
> > > @@ -75,6 +75,8 @@ static const int user_feature_bits[] = {
> > >      VIRTIO_NET_F_MTU,
> > >      VIRTIO_F_IOMMU_PLATFORM,
> > >  
> > > +    VIRTIO_NET_F_CTRL_VLAN,
> > > +
> > >      /* This bit implies RARP isn't sent by QEMU out of band */
> > >      VIRTIO_NET_F_GUEST_ANNOUNCE,
> > >  
> > > -- 
> > > 2.17.1