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Date:   Tue, 2 Mar 2021 13:08:12 +0100
From:   Cornelia Huck <cohuck@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Si-Wei Liu <si-wei.liu@oracle.com>, elic@nvidia.com,
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        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        virtio-dev@lists.oasis-open.org
Subject: Re: [virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow
 reset to zero
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On Mon, 1 Mar 2021 11:51:08 +0800
Jason Wang <jasowang@redhat.com> wrote:

> On 2021/3/1 5:25 =E4=B8=8A=E5=8D=88, Michael S. Tsirkin wrote:
> > On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote: =20
> >> On 2021/2/26 2:53 =E4=B8=8A=E5=8D=88, Michael S. Tsirkin wrote: =20

> >>> Confused. What is wrong with the above? It never reads the
> >>> field unless the feature has been offered by device. =20
> >>
> >> So the spec said:
> >>
> >> "
> >>
> >> The following driver-read-only field, max_virtqueue_pairs only exists =
if
> >> VIRTIO_NET_F_MQ is set.
> >>
> >> "
> >>
> >> If I read this correctly, there will be no max_virtqueue_pairs field i=
f the
> >> VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() violat=
es
> >> what spec said.
> >>
> >> Thanks =20
> > I think that's a misunderstanding. This text was never intended to
> > imply that field offsets change beased on feature bits.
> > We had this pain with legacy and we never wanted to go back there.
> >
> > This merely implies that without VIRTIO_NET_F_MQ the field
> > should not be accessed. Exists in the sense "is accessible to driver".
> >
> > Let's just clarify that in the spec, job done. =20
>=20
>=20
> Ok, agree. That will make things more eaiser.

Yes, that makes much more sense.

What about adding the following to the "Basic Facilities of a Virtio
Device/Device Configuration Space" section of the spec:

"If an optional configuration field does not exist, the corresponding
space is still present, but reserved."

(Do we need to specify what a device needs to do if the driver tries to
access a non-existing field? We cannot really make assumptions about
config space accesses; virtio-ccw can just copy a chunk of config space
that contains non-existing fields... I guess the device could ignore
writes and return zeroes on read?)

I've opened https://github.com/oasis-tcs/virtio-spec/issues/98 for the
spec issues.