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Jorge Moreira <jemoreira@google.com> pointed out that the ring state machine is underspecified. In the discussion that followed, we discovered that the spec says one thing and implementations do something else. This patch updates the spec to reflect how things are actually implemented across widely-used front-ends and back-ends including QEMU, crosvm, rust-vmm, and DPDK. Do this while taking care not to make any other existing implementations non-compliant by changing the spec. The spec says rings are started when a kick is received but the implementations actually start rings when VHOST_USER_SET_VRING_KICK is received. Reconcile this as follows: - Clarify that a ring can be stopped and then started again. The back-end must resume processing available requests when the ring is restarted. - Update the spec to say rings are started when VHOST_USER_SET_VRING_KICK is received. - Ensure compatibility by saying front-ends SHOULD inject a kick in case the back-end strictly implemented the old spec. - Avoid future back-end dependencies on injected kicks by saying that back-ends SHOULD NOT expect a kick to start rings. This way implementors have clarity on how things work while still allowing compatibility for existing implementations. Reported-by: Jorge Moreira <jemoreira@google.com> Cc: "Michael S . Tsirkin" <mst@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Stefano Garzarella <sgarzare@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Message-ID: <20260604201029.250450-2-stefanha@redhat.com>