Re: [PATCH RFC v2 0/4] virtio-net: add tx-hash, rx-tstamp, tx-tstamp and tx-time

On Mon, Feb 8, 2021 at 1:56 PM Willem de Bruijn
[off-list ref] wrote:

From: Willem de Bruijn <willemb@google.com>

RFCv2 for four new features to the virtio network device:

1. pass tx flow state to host, for routing + telemetry
2. pass rx tstamp to guest, for better RTT estimation
3. pass tx tstamp to guest, idem
3. pass tx delivery time to host, for accurate pacing

All would introduce an extension to the virtio spec.
Concurrently with code review I will write ballots to
https://www.oasis-open.org/committees/ballots.php?wg_abbrev=virtio

These changes are to the driver side. Evaluation additionally requires
achanges to qemu and at least one back-end. I implemented preliminary
support in Linux vhost-net. Both patches available through github at

https://github.com/wdebruij/linux/tree/virtio-net-txhash-2
https://github.com/wdebruij/qemu/tree/virtio-net-txhash-2

Changes RFC -> RFCv2
  - add transmit timestamp patch
  - see individual patches for other changes

Willem de Bruijn (4):
  virtio-net: support transmit hash report
  virtio-net: support receive timestamp
  virtio-net: support transmit timestamp
  virtio-net: support future packet transmit time

Seeing Yuri's patchset adding new features reminded me that I did not
follow-up on this patch series on the list.

The patches themselves are mostly in good shape. The last tx tstamp
issue can be resolved.

But the device implementation I target only supports legacy mode.
Below conversation that we had in one of the patches makes clear that
supporting this in legacy is not feasible. Nor is upgrading that
device in the short term. Until there is a device implementation that
implements these offloads, these features are a dead letter. Not moving
forward for now.

Somewhat related: is there a plan for when we run out of 64 feature bits?

quoted

quoted

Actually, would it be possible to make new features available on
legacy devices? There is nothing in the features bits precluding it.

I think it won't be possible: you are using feature bit 55,
legacy devices have up to 32 feature bits. And of course the
header looks a bit differently for legacy, you would have to add special
code to handle that when mergeable buffers are off.

I think I can make the latter work. I did start without a dependency
on the v1 header initially.

Feature bit array length I had not considered. Good point. Need to
think about that. It would be very appealing if in particular the
tx-hash feature could work in legacy mode.