virtio: Re-enable notifications after drain
During drain, we do not care about virtqueue notifications, which is why
we remove the handlers on it. When removing those handlers, whether vq
notifications are enabled or not depends on whether we were in polling
mode or not; if not, they are enabled (by default); if so, they have
been disabled by the io_poll_start callback.
Because we do not care about those notifications after removing the
handlers, this is fine. However, we have to explicitly ensure they are
enabled when re-attaching the handlers, so we will resume receiving
notifications. We do this in virtio_queue_aio_attach_host_notifier*().
If such a function is called while we are in a polling section,
attaching the notifiers will then invoke the io_poll_start callback,
re-disabling notifications.
Because we will always miss virtqueue updates in the drained section, we
also need to poll the virtqueue once after attaching the notifiers.
Buglink: https://issues.redhat.com/browse/RHEL-3934
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20240202153158.788922-3-hreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/include/block/aio.h b/include/block/aio.h
index 5d0a114..8378553 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -480,9 +480,14 @@
AioPollFn *io_poll,
EventNotifierHandler *io_poll_ready);
-/* Set polling begin/end callbacks for an event notifier that has already been
+/*
+ * Set polling begin/end callbacks for an event notifier that has already been
* registered with aio_set_event_notifier. Do nothing if the event notifier is
* not registered.
+ *
+ * Note that if the io_poll_end() callback (or the entire notifier) is removed
+ * during polling, it will not be called, so an io_poll_begin() is not
+ * necessarily always followed by an io_poll_end().
*/
void aio_set_event_notifier_poll(AioContext *ctx,
EventNotifier *notifier,
