[PATCH RFC v2 0/8] vsock: add datagram support to virtio and vhost

[Sergio Lopez <slp@redhat.com>]

This RFC adds SOCK_DGRAM support to virtio-vsock and vhost-vsock through
the existing AF_VSOCK API. It is intended for guest and host services
that exchange independent requests, responses, or events and need message
boundaries and sender addresses without a stream connection or application
framing.

VIRTIO_VSOCK_F_DGRAM gates virtio traffic in both directions. Each virtio
send carries zero to 64 KiB of payload in one VIRTIO_VSOCK_TYPE_DGRAM
packet. On the RX virtqueue, the device may split a datagram across
page-sized guest buffers.
START and EOM delimit the message; the guest reassembles at most 64
fragments and 64 KiB before delivering it to a socket. Invalid or
incomplete messages are discarded. Datagram delivery is best effort:
there are no peer credits, acknowledgements, or retransmissions. Send
admission uses the socket send-memory limit, and SIOCOUTQ reports charged
send memory.

Datagram sockets have a separate port space from stream and seqpacket
sockets. The latter two continue to share their port space. Datagram
routes are chosen per destination among guest-to-host, host-to-guest, and
loopback transports, with VMCI as a fallback. VMCI still owns its socket
handle when it is present; loopback works without a directional transport.
Hyper-V continues to reject datagram sockets.

Bobby Eshleman and Amery Hung explored this feature in their v6 series
and v7 work-in-progress branch. This series is a fresh implementation
against the current vsock code. It follows the separate datagram port
space, per-destination routing, and VMCI fallback requirements discussed
there. For guest receive buffers, it uses bounded fragmentation and
reassembly rather than full-message buffers or the earlier drop/error
queue approach:

https://lore.kernel.org/r/20240710212555.1617795-1-amery.hung@bytedance.com
https://github.com/ameryhung/linux/tree/vsock-dgram-v7

A corresponding virtio specification update has been drafted separately.
The wire format and feature negotiation still need specification review
before this series can be considered for merging.

The first patches expose the core port allocator and add datagram queue
reporting, then implement the common data path, guest and host feature
negotiation, routing, and RX fragmentation. The last patch extends
vsock_test with local and peer tests for message boundaries, routing,
connected peers, port sharing, queue accounting, and delayed reads.

Testing used QEMU with the F_DGRAM property patch below, an x86-64
KASAN/lockdep kernel, paired guest tests, and a nested Linux vhost-vsock
backend. The tested paths include 64 KiB messages, delayed-read bursts,
feature-disabled ingress, stream and seqpacket controls, and header-only
guest RX buffers. A temporary userspace backend still fails stream test
21 on both the series base and tip; that case passes with Linux vhost.
Real VMCI and Hyper-V hardware were not exercised.

https://lists.gnu.org/archive/html/qemu-devel/2026-10/msg08074.html

Code was written and reviewed both manually and assisted by Codex
(GPT-6.1-Sol) and Claude (Opus 4.6).

Signed-off-by: Sergio Lopez <slp@redhat.com>
---
Changes in v2:
- Keep guest RX buffers page-sized; add START/EOM framing and bounded
  datagram reassembly (thanks to Alexei Starovoitov and Bobby Eshleman).
- Preserve stream and seqpacket data when an RX buffer has no payload
  room.
- Separate datagram and connection-oriented port spaces, and share
  receive metadata and queueing (thanks to Alexei Starovoitov).
- Check negotiated support and destination CID on ingress, and require
  an exact connected peer CID and port (thanks to Sashiko AI bot).
- Select a capable route for each destination, retain VMCI as fallback,
  and support loopback-only sockets (thanks to Alexei Starovoitov).
- Preserve the connected peer address across temporary parent fallback
  (thanks to Sashiko AI bot).
- Use socket write-memory accounting for datagram SIOCOUTQ instead of
  separate transport counters (thanks to Alexei Starovoitov).
- Extend selftests for fragmentation, delayed reads, port sharing,
  connected peers, and capability-based skips; fix the test build and
  README entries (thanks to Alexei Starovoitov, Bobby Eshleman,
  and Sashiko AI bot).
- Link to v1: https://lore.kernel.org/r/20261028-vsock-dgram-support-v1-0-e457cf7cee78@sinrega.org

---
Sergio Lopez (8):
      vsock: expose the core port allocator to transports
      vsock: report datagram receive and send queue state
      vsock/virtio: implement the datagram data path
      vsock/virtio: negotiate datagram support on the guest
      vhost/vsock: negotiate datagram support with guests
      vsock: allow datagrams to use directional transports
      vsock: fragment datagrams across receive buffers
      vsock: add datagram tests to vsock_test

 Documentation/admin-guide/sysctl/net.rst |    8 +-
 drivers/vhost/vsock.c                    |   51 +-
 include/linux/virtio_vsock.h             |    9 +-
 include/net/af_vsock.h                   |   30 +-
 include/uapi/linux/virtio_vsock.h        |   10 +
 net/vmw_vsock/af_vsock.c                 |  459 +++++++++++---
 net/vmw_vsock/hyperv_transport.c         |   11 +-
 net/vmw_vsock/virtio_transport.c         |  167 ++++-
 net/vmw_vsock/virtio_transport_common.c  |  164 ++++-
 net/vmw_vsock/vmci_transport.c           |  115 ++--
 net/vmw_vsock/vsock_bpf.c                |    3 +-
 net/vmw_vsock/vsock_loopback.c           |    2 +-
 tools/testing/vsock/Makefile             |    3 +-
 tools/testing/vsock/README               |   31 +-
 tools/testing/vsock/util.c               |   62 +-
 tools/testing/vsock/util.h               |   10 +
 tools/testing/vsock/vsock_test.c         |  175 +++++-
 tools/testing/vsock/vsock_test_dgram.c   | 1000 ++++++++++++++++++++++++++++++
 tools/testing/vsock/vsock_test_dgram.h   |   29 +
 19 files changed, 2066 insertions(+), 273 deletions(-)
---
base-commit: 65ba8f34ec886ec72b2c07ddc8cf3d815c32869d
change-id: 20261027-vsock-dgram-support-683a4e0aebe7

Best regards,
-- 
Sergio Lopez <slp@redhat.com>