linux-aio: poll ring for completions

The Linux AIO userspace ABI includes a ring that is shared with the
kernel.  This allows userspace programs to process completions without
system calls.

Add an AioContext poll handler to check for completions in the ring.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-id: 20161201192652.9509-6-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/linux-aio.c

@@ -255,6 +255,20 @@ static void qemu_laio_completion_cb(EventNotifier *e)
     }
 }
 
+static bool qemu_laio_poll_cb(void *opaque)
+{
+    EventNotifier *e = opaque;
+    LinuxAioState *s = container_of(e, LinuxAioState, e);
+    struct io_event *events;
+
+    if (!io_getevents_peek(s->ctx, &events)) {
+        return false;
+    }
+
+    qemu_laio_process_completions_and_submit(s);
+    return true;
+}
+
 static void laio_cancel(BlockAIOCB *blockacb)
 {
     struct qemu_laiocb *laiocb = (struct qemu_laiocb *)blockacb;
@@ -448,7 +462,8 @@ void laio_attach_aio_context(LinuxAioState *s, AioContext *new_context)
     s->aio_context = new_context;
     s->completion_bh = aio_bh_new(new_context, qemu_laio_completion_bh, s);
     aio_set_event_notifier(new_context, &s->e, false,
-                           qemu_laio_completion_cb, NULL);
+                           qemu_laio_completion_cb,
+                           qemu_laio_poll_cb);
 }
 
 LinuxAioState *laio_init(void)