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vhost-user.rst: clarify when FDs can be sent

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Previously the spec did not say where in a message the FDs should be
sent.  As I understand it, FDs transferred in ancillary data will
always be received along with the first byte of the data they were
sent with, so we should define which byte that is.  Going by both
libvhost-user in QEMU and the rust-vmm crate, that byte is the first
byte of the message header.  This is important to specify because it
would make back-end implementation significantly more complicated if
receiving file descriptors in the middle of a message had to be
handled.

Signed-off-by: Alyssa Ross <hi@alyssa.is>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20251106192105.3456755-1-hi@alyssa.is>
This commit is contained in:
Alyssa Ross 2025-11-06 20:21:05 +01:00 • committed by Michael S. Tsirkin
parent fa41363879
commit 97f24a0496
1 changed files with 7 additions and 0 deletions
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7
docs/interop/vhost-user.rst
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@ -411,6 +411,13 @@ in the ancillary data:
* ``VHOST_USER_SET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
* ``VHOST_USER_SET_DEVICE_STATE_FD``
When sending file descriptors in ancilliary data, *front-end* should
associate the ancilliary data with a ``sendmsg`` operation (or
equivalent) that sends bytes starting with the first byte of the
message header. *back-end* can therefore expect that file descriptors
will only be received in the first ``recvmsg`` operation for a message
header.
If *front-end* is unable to send the full message or receives a wrong
reply it will close the connection. An optional reconnection mechanism
can be implemented.