RE: [PATCH v2 1/2] virtio-net: Fix receive buffer size calculation text

[Parav Pandit <parav@nvidia.com>]

Hi Cornelia,

> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Tuesday, January 16, 2024 6:59 PM
> 
> On Tue, Jan 16, 2024 at 01:18:59PM +0000, Parav Pandit wrote:
> >
> > > From: Cornelia Huck <cohuck@redhat.com>
> > > Sent: Tuesday, January 16, 2024 6:08 PM
> > > To: Parav Pandit <parav@nvidia.com>;
> > > virtio-comment@lists.oasis-open.org;
> > > mst@redhat.com
> > > Cc: Shahaf Shuler <shahafs@nvidia.com>; xuanzhuo@linux.alibaba.com;
> > > yuri.benditovich@daynix.com
> > > Subject: RE: [PATCH v2 1/2] virtio-net: Fix receive buffer size
> > > calculation text
> > >
> > > On Tue, Jan 16 2024, Parav Pandit <parav@nvidia.com> wrote:
> > >
> > > >> From: Cornelia Huck <cohuck@redhat.com>
> > > >> Sent: Tuesday, January 16, 2024 4:33 PM
> > > >> To: Parav Pandit <parav@nvidia.com>;
> > > >> virtio-comment@lists.oasis-open.org;
> > > >> mst@redhat.com
> > > >> Cc: Shahaf Shuler <shahafs@nvidia.com>;
> > > >> xuanzhuo@linux.alibaba.com; yuri.benditovich@daynix.com
> > > >> Subject: RE: [PATCH v2 1/2] virtio-net: Fix receive buffer size
> > > >> calculation text
> > > >>
> > > >> On Tue, Jan 16 2024, Parav Pandit <parav@nvidia.com> wrote:
> > > >>
> > > >> >> From: Cornelia Huck <cohuck@redhat.com>
> > > >> >> Sent: Monday, January 15, 2024 10:14 PM
> > > >> >
> > > >> >> On Mon, Jan 15 2024, Parav Pandit <parav@nvidia.com> wrote:
> > > >> >> > +The driver MUST consider size of field \field{struct
> > > >> >> > +virtio_net_hdr}
> > > >> >> > +20 bytes if VIRTIO_NET_F_HASH_REPORT is negotiated, and 12
> > > >> >> > +bytes if
> > > >> >> not.
> > > >> >> > +
> > > >> >>
> > > >> >> Requiring the driver to consider the size of something to be
> > > >> >> its actual size seems a bit odd :) I don't think we need this,
> > > >> >> as the length can be derived from looking at the definitions,
> > > >> >> and is already spelled out explicitly, if you consider my suggestion
> above.
> > > >> > We need this because tx side also needs to refer to the
> > > >> > virtio_net_hdr in
> > > >> patch 2 to be same as that of the rx side.
> > > >> > And hence, this normative sets base line for tx side too.
> > > >> > Relying on rest of the
> > > >> receive packet normative is not enough.
> > > >>
> > > >> Hm, why? If struct virtio_net_hdr is well-defined, its size is
> > > >> well-defined as well, and we do not need to state it explictly?
> > > > Because,
> > > > the size of virtio_net_hdr is derived from the rx side features.
> > > > Today there is no normative line that says that even though you
> > > > are using A,
> > > B, C Rx features, due to which your tx side virtio_net_hdr also changes.
> > > > The 2nd patch in this series adds this explicit normative as
> > > > explained in the
> > > cover letter.
> > >
> > > Let's step back a bit.
> > >
> > > struct virtio_net_hdr is defined at the beginning of the "Device Operation"
> > > section; the definition clearly says that the last three fields
> > > depend on VIRTIO_NET_F_HASH_REPORT being negotiated. The device
> and
> > > the driver agree on whether HASH_REPORT is negotiated, and therefore
> > > should also agree on the size of virtio_net_hdr?
> > >
> > Do you imply that device operation description is enough to not add
> normative?
> > If so, for this case and possibly new things if we write as device operation,
> would it be enough?
> >
> > > Or is the problem that we did not state explicitly that the last
> > > three fields of virtio_net_hdr do not exist without HASH_REPORT (and
> > > are not merely invalid)? If yes, we should spell this out, instead
> > > of adding normative statements about what the size of virtio_net_hdr
> should be considered to be.
> > This suggested normative is added in this patch.
> >
> > > If virtio_net_hdr has a fixed size, we shouldn't need the second patch,
> either.
> > The fact that HASH_REPORT is only for the rx, if we have to go back in time,
> there is no need for the tx to force also to follow the rx virtio_net_hdr.
> > There is no explicit normative indicating the virtio_net_hdr for the TX is
> forced by the RX even though it has no relation to hash report.
> >
> > If you say device operation is enough, than I am sort of lost of when
> normative is needed, and when device operation is enough.
> 
> 
> Generally we start adding normatives when we see that something is unclear.
> But I think generally I agree with Parav, if someone has the time to write the
> normative, it's all good.

Are you ok with the v3?
I kept the clarification for tx and addressed rest of your and Xuan's comments in it.
https://lists.oasis-open.org/archives/virtio-comment/202401/msg00070.html