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

[Steffen Trumtrar]

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:

 - 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. The driver reports it as
   SOF_TIMESTAMPING_RAW_HARDWARE without a PTP clock (phc_index -1),
   so userspace cannot tell which clock it refers to and may compare
   it against CLOCK_REALTIME (currently off by 37s). Options I see:
   convert to the guest's system time in the driver (correct only if
   host and guest agree on the TAI offset), expose a PHC so the
   time base is explicit, or keep the raw TAI value and leave the
   conversion to userspace. Which one is preferred?
 - Like the existing rx_hash, the XDP timestamp callback locates the
   header relative to xdp->data, so it must be called before
   bpf_xdp_adjust_head().

A virtio spec change is needed as well and not yet proposed.

Testing: the feature needs a QEMU that knows VIRTIO_NET_F_RX_TSTAMP and
sets the matching vnet header size; see
https://github.com/strumtrar/qemu/tree/v11.0.1/virtio-rx-stamps

Performance: I measured the receive rate with 64 byte UDP packets in
a nested QEMU/KVM setup with vhost-net. The variance between runs is
larger than any difference between the variants (without the series,
series without the feature, feature negotiated, feature enabled), so
I cannot show a measurable cost or the absence of one. Numbers from
bare metal will follow with a non-RFC version.

Signed-off-by: Steffen Trumtrar <s.trumtrar@xxxxxxxxxxxxxx>
---
Changes in v4:
- zero the header beyond the known fields on tx and in vhost
- grow padded_vnet_hdr for the 40 byte header (big packets)
- install xdp_metadata_ops if only RX_TSTAMP is negotiated
- rename VIRTIO_NET_F_TSTAMP to VIRTIO_NET_F_RX_TSTAMP
- move tstamp to offset 32 (header size 40) to match virtio spec layout
- drop XDP restriction for SIOCSHWTSTAMP, add xmo_rx_timestamp
- add static_asserts for header sizes
- add support in vhost
- move virtio_net_common_hdr to virtio_net.h
- Link to v3: https://patch.msgid.link/20260921-v6-7-topic-virtio-net-ptp-v3-0-3350851c4f88@xxxxxxxxxxxxxx
Changes in v3:
- rework to use virtio_net_common_hdr instead of adding new struct
Changes in v2:
- rework patches to use flow filter instead of feature flag
- Link to v1: https://lore.kernel.org/r/20231218-v6-7-topic-virtio-net-ptp-v1-0-cac92b2d8532@xxxxxxxxxxxxxx

To: Michael S. Tsirkin <mst@xxxxxxxxxx>
To: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
To: David S. Miller <davem@xxxxxxxxxxxxx>
To: Jakub Kicinski <kuba@xxxxxxxxxx>
To: Paolo Abeni <pabeni@xxxxxxxxxx>
To: Richard Cochran <richardcochran@xxxxxxxxx>
To: Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>
To: Andrew Lunn <andrew+netdev@xxxxxxx>
To: Eugenio Pérez <eperezma@xxxxxxxxxx>
To: Jason Wang <jasowangio@xxxxxxxxx>
To: Alexei Starovoitov <ast@xxxxxxxxxx>
To: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
To: Jesper Dangaard Brouer <hawk@xxxxxxxxxx>
To: John Fastabend <john.fastabend@xxxxxxxxx>
To: Stanislav Fomichev <sdf@xxxxxxxxxxx>
To: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: virtualization@xxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: bpf@xxxxxxxxxxxxxxx
Cc: kvm@xxxxxxxxxxxxxxx

---
Steffen Trumtrar (6):
      virtio-net: move virtio_net_common_hdr to virtio_net.h
      virtio-net: support receive timestamp
      virtio-net: assert header sizes
      vhost-net: support VIRTIO_NET_F_RX_TSTAMP
      virtio-net: add XDP rx timestamp metadata
      tun: add rx timestamp to the vnet header

 drivers/net/tun.c               |  14 ++--
 drivers/net/tun_vnet.h          |  18 +++--
 drivers/net/virtio_net.c        | 171 ++++++++++++++++++++++++++++++++++------
 drivers/vhost/net.c             |  17 +++-
 include/linux/virtio_net.h      |  30 +++++++
 include/uapi/linux/virtio_net.h |   1 +
 6 files changed, 214 insertions(+), 37 deletions(-)
---
base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222
change-id: 20231218-v6-7-topic-virtio-net-ptp-3df023bc4f4d

Best regards,
--  
Steffen Trumtrar <s.trumtrar@xxxxxxxxxxxxxx>