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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Steffen Trumtrar wrote:
> This series tries to pick up the work on the virtio-net timestamping
> feature from Willem de Bruijn.
> 
> Original series
>     Message-Id: 20210208185558.995292-1-willemdebruijn.kernel@gmail.com
>     Subject: [PATCH RFC v2 0/4] virtio-net: add tx-hash, rx-tstamp,
>     tx-tstamp and tx-time
>     From: Willem de Bruijn <willemb@google.com>
> 
>     RFC 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.
> 
> The original series consisted of a hack around the DMA API, which should
> be fixed in this series.
> 
> The changes in this series are to the driver side. For the changes to qemu see:
>     https://github.com/strumtrar/qemu/tree/v8.1.1/virtio-net-ptp
> 
> Currently only virtio-net is supported. The original series used
> vhost-net as backend. However, the path through tun via sendmsg doesn't
> allow us to write data back to the driver side without any hacks.
> Therefore use the way via plain virtio-net without vhost albeit better
> performance.
> 
> Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>

Thanks for picking this back up, Steffen. Nice to see that the code still
applies mostly cleanly.

For context: I dropped the work only because I had no real device
implementation. The referenced patch series to qemu changes that.

I suppose the main issue is the virtio API changes that this introduces,
which will have to be accepted to the spec.

One small comment to patch 4: there I just assumed the virtual device
time is CLOCK_TAI. There is a concurrent feature under review for HW
pacing offload with AF_XDP sockets. The clock issue comes up a bit. In
general, for hardware we cannot assume a clock. For virtio, perhaps
assuming the same monotonic hardware clock in guest and host can be
assumed. But this clock alignment needs some thought.