RE: [virtio-comment] [PATCH 2/2] virtio-net: Clarify the size of the struct virtio_net_hdr for tx

[Parav Pandit <parav@nvidia.com>]

> From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Sent: Wednesday, December 13, 2023 11:33 AM
> 
> On Fri, 10 Nov 2023 14:40:40 +0200, Parav Pandit <parav@nvidia.com>
> wrote:
> > The feature VIRTIO_NET_F_HASH_REPORT only applies to the receive side.
> > However, when VIRTIO_NET_F_HASH_REPORT feature was introduced, it
> was
> > not clarified that the size of the struct virtio_net_hdr on the packet
> > transmission also uses higher size when VIRTIO_NET_F_HASH_REPORT is
> > negotiated.
> >
> > Explicitly clarify this.
> >
> > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/183
> > Signed-off-by: Parav Pandit <parav@nvidia.com>
> > ---
> >  device-types/net/description.tex | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/device-types/net/description.tex
> > b/device-types/net/description.tex
> > index f5647d4..00ea58d 100644
> > --- a/device-types/net/description.tex
> > +++ b/device-types/net/description.tex
> > @@ -531,6 +531,12 @@ \subsubsection{Packet
> > Transmission}\label{sec:Device Types / Network Device / De
> >
> >  \drivernormative{\paragraph}{Packet Transmission}{Device Types /
> > Network Device / Device Operation / Packet Transmission}
> >
> > +If VIRTIO_NET_F_HASH_REPORT is not negotiated, the size of the field
> > +\field{struct virtio_net_hdr} is 12 bytes.
> > +
> > +If VIRTIO_NET_F_HASH_REPORT is negotiated, the size of the field
> > +\field{struct virtio_net_hdr} is 20 bytes.
> > +
> 
> If we need a new feature, because that the implement of the current linux
> kernel has the same size on the tx patch.

I didn't understand your comment -  "if we need a new feature" part.

Current Linux kernel has same size for tx and rx, snippet below.

        if (vi->has_rss_hash_report)
                vi->hdr_len = sizeof(struct virtio_net_hdr_v1_hash);

So if you are asking if we need a new feature bit? I believe, we don't need a new feature bit for current implementations and behavior.

We will new feature bit in future to have shorter virtio_net_hdr on tx, however we better consume it as part of new inline descriptor format.