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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 13, 2026 at 01:47:33PM +0100, Sergio Lopez wrote:
> 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.

where do these limits come from? how does peer know not to send more?

Thanks!

> 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>