Re: [PATCH RFC v2 7/8] vsock: fragment datagrams across receive buffers

[sashiko-bot@kernel.org]

> The guest already uses page-sized RX allocations. A datagram can exceed
> one RX buffer. Growing every posted buffer to hold a full datagram would
> increase virtqueue memory use and make small queued datagrams retain large
> backing allocations. With delayed reads, a bounded receive queue could
> then fill after only a few messages.
> 
> Define START and EOM flags for datagram packets. Mark outgoing messages
> with both flags, and let vhost split a message across guest RX buffers,
> keeping START on the first fragment and EOM on the last. When a data
> packet meets a descriptor with no payload space, vhost returns that
> descriptor with zero used bytes. It drops datagrams but requeues stream
> and seqpacket data for a later buffer.
> 
> Reassemble consecutive fragments in the guest before tap or socket
> delivery. Bound a partial message to 64 fragments and 64 KiB. Discard
> incomplete messages on malformed packets, a new START, a peer mismatch,
> an intervening non-datagram packet, or virtqueue teardown. Require complete
> framing for host and loopback ingress.
> 
> A QEMU reproducer now delivers a 9 KiB datagram as one message and all
> 16 messages of a delayed-read burst. Paired host/guest selftests cover
> 64 KiB messages and the burst through both a userspace backend and the
> patched Linux vhost backend, with stream and seqpacket controls. A nested
> vhost-vsock guest posting header-only RX buffers also passes stream and
> seqpacket receive tests.
> 
> Signed-off-by: Sergio Lopez <slp@redhat.com>

Sashiko has reviewed this patch and found no issues.

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