tree dd680789368f78aae6fcfb521fe55d868eff3b5f
parent 3b720b8c865098c49c1570b6b5c7832bcfa6e6c2
author Rusty Russell <rusty@rustcorp.com.au> 1326345283 +1030
committer Rusty Russell <rusty@rustcorp.com.au> 1326345283 +1030

virtio: expose added descriptors immediately.

A virtio driver does virtqueue_add_buf() multiple times before finally
calling virtqueue_kick(); previously we only exposed the added buffers
in the virtqueue_kick() call.  This means we don't need a memory
barrier in virtqueue_add_buf(), but it reduces concurrency as the
device (ie. host) can't see the buffers until the kick.

In the unusual (but now possible) case where a driver does add_buf()
and get_buf() without doing a kick, we do need to insert one before
our counter wraps.  Otherwise we could wrap num_added, and later on
not realize that we have passed the marker where we should have
kicked.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>