RE: [virtio-dev] Re: [PATCH v3] virtio-net: Mention VIRTIO_NET_F_HASH_REPORT dependency on VIRTIO_NET_F_CTRL_VQ

[Parav Pandit <parav@nvidia.com>]

> From: Alvaro Karsz <alvaro.karsz@solid-run.com>
> Sent: Saturday, February 11, 2023 4:13 AM


> > Why cannot device say as MUST requirement?
> >
> > Let say there is a device out that exposes a F_HASH_REPORT without
> F_CTRL_VQ.
> > So driver tried to send a command and failed to issue cmd.
> > End result, hash config was not successful.
> > Did driver gain anything from seeing supplied hash in the config space?
> > No. it just confused driver that device offered feature bit, could see the
> supposed hash, but still couldn't configure it.
> >
> > So, I think the right fix is that device MUST NOT set F_HASH_REPORT without
> F_CTRL_VQ.
> >
> > And driver should .. is fine, because existing driver that followed 1.2 will
> negotiate, and device will also accept it if it offered without F_CTRL_VQ.
> >
> > Any new device that follows 1.3 will not offer F_HAS_REPORT without C_VQ,
> hence it cannot be negotiated by driver without C_VQ.
> >
> > The gain is : device doesn't need to continue offering F_HASH_REPORT
> without C_VQ. And I doubt if any device would have done it, as it was obvious.
> > Just the spec was missed out.
> >
> > Is MUST/SHOULD big deal here for device? At least not to me, from practical
> standpoint to me.
> > Making must just makes the spec consistent with rest without breaking
> backward compat.
> 
> The first version of this patch [1] actually used a bit requirement (which is
> equivalent to a "MUST" if I understand correctly).
> Then Michael pointed out that we can't do it since a version was published
> without this requirement.
> 
> It's clear that the control VQ is required to use this feature.
> In the linux kernel implementation probe will fail if a device offers
> F_HASH_REPORT without F_CTRL_VQ.
> 
This is even better.
> Ideally we'll add a "MUST", but since we can't, 
Lets hear Michael's view, why MUST cannot be done.
Based on our discussion here, I think MUST is possible and cleaner without breaking any existing sw or device.

> our options are to have a
> "SHOULD", or not mention the dependency at all.
> 
> [1] https://lists.oasis-open.org/archives/virtio-comment/202302/msg00026.html