Re: [PATCH] vhost-user.rst: clarify when FDs can be sent

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

On Thu, Nov 06, 2025 at 08:21:05PM +0100, Alyssa Ross wrote:
> Previously the spec did not say where in a message the FDs should be
> sent.  As I understand it, FDs transferred in ancilliary data will

ancillary, actually

> 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>
> ---
>  docs/interop/vhost-user.rst | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index 2e50f2ddfa..93a9c8df2b 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -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

ancillary, here too

> +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.
> 
> base-commit: 917ac07f9aef579b9538a81d45f45850aba42906
> -- 
> 2.51.0