Re: [PATCH RFC v4 0/6] virtio-net: add receive timestamp support

[Willem de Bruijn]

Steffen Trumtrar wrote:
> 
> Hi Willem,
> 
> On 2026-11-01 at 09:57 -05, Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote:
> 
> > Steffen Trumtrar wrote:
> > > This series picks up Willem de Bruijn's work on a virtio-net receive
> > > timestamp. The device puts the time of arrival (TAI, ns) into the
> > > virtio-net header so the guest can use it for PTP and RTT estimation
> > > without taking the timestamp itself after the host-to-guest latency.
> > > 
> > > The series is an RFC. I am mostly interested in feedback on the
> > > approach and on the wire format:
> > 
> > At a high level +1 from me.
> 
> Thanks!
> 
> > 
> > Some comments inline.
> > > 
> > >  - The timestamp is a new field at the end of virtio_net_common_hdr
> > >    after outer_nh_offset and ipsec, offset 32 (see virtio spec),
> > >    negotiated with VIRTIO_NET_F_RX_TSTAMP (bit 49). Header size now 40
> > >    bytes.
> > >    Willem noted on v3 that virtio_net_common_hdr was not meant to be
> > >    extended this way; I kept it as the simplest approach, but a
> > >    separate struct for the timestamp header is an option.
> > >  - Timestamping is enabled at runtime with SIOCSHWTSTAMP; all
> > >    supported rx filters behave the same for now.
> > >  - XDP programs can read the timestamp with
> > >    bpf_xdp_metadata_rx_timestamp().
> > >  - vhost-net passes the feature through; the timestamp itself is
> > >    written by tun. The tun patch is only meant as a reference
> > >    implementation for the device side.
> > >  - The timestamp is in TAI.
> > 
> > Is this a requirement? It is common for PHCs to be synced to TAI,
> > but not 100% the case. Does virtio need to specify this.
> >
> 
> No, I don't think it has to be TAI. TAI is only what the tun reference
> implementation does (ktime_get_clocktai_ns()). What the guest really
> needs is to know which clock the timestamp comes from, so it can compare
> it with something.
> 
> One idea for the spec change: don't fix the time base in virtio-net at
> all, only say the timestamp is in nanoseconds of some device clock, and
> let the device tell the driver which clock that is. virtio_rtc already
> has clock ids with a type (UTC, TAI, monotonic, ...), and the Linux
> driver registers a PHC for each of them (with VIRTIO_RTC_PTP).
> virtio-net could carry the virtio_rtc clock id in its config space, and
> the driver reports the index of that PHC in get_ts_info. That would also
> fix the phc_index == -1 problem you pointed out in 2/6.
> 
> What I don't like about it: virtio-net then depends on a second device
> being there, and with more than one virtio_rtc device the clock id alone
> is not enough to find the right one. Without virtio_rtc the driver would
> not offer hardware timestamps at all.
> 
> The alternative is a PHC in virtio-net itself (gettime over the control
> vq), which is a lot more spec for one timestamp field.
> 
> Do you think the virtio_rtc way is reasonable, or is there something
> simpler I'm missing?

I don't have an obvious answer. This needs consensus.

Hardcoding clock base to TAI is definitely easiest.

Outside of virtio, the relationship between PHY RX timestamps and PHC
is well understood. It would make some sense for virtio to follow that
model.

But indeed when having multiple NICs and multiple PHCs, how to map them
I think is an unresolved issue in general? Of course, aslo moot if they
are all synced, which I think may be an implicit assumption especially
in the world of spec compliant PTP.

And, if virtualizing anyway, it is feasible to do the clock conversion
in the software device, not having to bother with all this complexity
inside the VM. Though maybe not for direct HW virtio-net queues.