From: Alyssa Ross <[email protected]> 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 <[email protected]> Reviewed-by: Michael S. Tsirkin <[email protected]> Signed-off-by: Michael S. Tsirkin <[email protected]> Message-Id: <[email protected]> --- 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 +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. -- MST
