RE: [virtio-comment] [PATCH v10] virtio-net: Clarify VLAN filter table configuration

[Parav Pandit <parav@nvidia.com>]

> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Wednesday, January 25, 2023 3:56 PM
> 
> On Wed, Jan 25, 2023 at 07:04:54PM +0100, Halil Pasic wrote:
> > > +
> > > +When VIRTIO_NET_F_CTRL_VLAN is negotiated, the device MUST accept
> > > +all VLAN tagged packets whose VLAN tag is present in the VLAN
> > > +filter table and SHOULD drop all VLAN tagged packets whose VLAN tag
> > > +is absent in the VLAN filter table.
> >
> > We could also add "as per device configuration" here as well...
> >
> > >From the other thread
> >
> > On Mon, 23 Jan 2023 12:41:16 +0000
> > Parav Pandit <parav@nvidia.com> wrote:
> >
> > > > BTW why SHOULD drop?
> > > This is the offload feature to drop such packets so that OS driver
> > > doesn't need to do the filtering work. :)
> >
> > Why not MUST drop? If the device is allowed to produce false
> > negatives, then I think the OS probably wants to check again to filter out
> those.
> > Can we change SHOULD to MUST, so the conforming device is guaranteed
> > to do the filtering, and the OS can rely on it?
> 
> Well the spec says:
> 	Similar to the MAC address based filtering, the VLAN filtering
> 	is also best-effort: unwanted packets could still arrive.
> 
>  am not sure why - IIRC MAC filtering is best effort because a. we don't expose
> table size to guests so the table can always overflow.
> b. we like to rely on bridge to filter for performance and *that* is
>    best effort.
> 
> but vlans are different aren't they? Anyway changing that will need a new
> feature bit.
I do not the history for about "Note" on why it was made best effort.
To best of my knowledge, it should not be best effort.
Device should not overflow the table. It should fail the ADD call.

If the above claim of "Note" is not strong enough, we can probably run a different issue to make it "MUST".
I am not sure we really need feature bit it we fix the note.