[PULL v4 40/47] event_notifier: add event_notifier_get_wfd()

["Michael S. Tsirkin" <mst@redhat.com>]

From: Sergio Lopez <slp@redhat.com>

event_notifier_get_fd(const EventNotifier *e) always returns
EventNotifier's read file descriptor (rfd). This is not a problem when
the EventNotifier is backed by a an eventfd, as a single file
descriptor is used both for reading and triggering events (rfd ==
wfd).

But, when EventNotifier is backed by a pipe pair, we have two file
descriptors, one that can only be used for reads (rfd), and the other
only for writes (wfd).

There's, at least, one known situation in which we need to obtain wfd
instead of rfd, which is when setting up the file that's going to be
sent to the peer in vhost's SET_VRING_CALL.

Add a new event_notifier_get_wfd(const EventNotifier *e) that can be
used to obtain wfd where needed.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20220304100854.14829-2-slp@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/qemu/event_notifier.h | 1 +
 util/event_notifier-posix.c   | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/include/qemu/event_notifier.h b/include/qemu/event_notifier.h
index b79add035d..8a4ff308e1 100644
--- a/include/qemu/event_notifier.h
+++ b/include/qemu/event_notifier.h
@@ -38,6 +38,7 @@ int event_notifier_test_and_clear(EventNotifier *);
 #ifdef CONFIG_POSIX
 void event_notifier_init_fd(EventNotifier *, int fd);
 int event_notifier_get_fd(const EventNotifier *);
+int event_notifier_get_wfd(const EventNotifier *);
 #else
 HANDLE event_notifier_get_handle(EventNotifier *);
 #endif
diff --git a/util/event_notifier-posix.c b/util/event_notifier-posix.c
index 8307013c5d..16294e98d4 100644
--- a/util/event_notifier-posix.c
+++ b/util/event_notifier-posix.c
@@ -99,6 +99,11 @@ int event_notifier_get_fd(const EventNotifier *e)
     return e->rfd;
 }
 
+int event_notifier_get_wfd(const EventNotifier *e)
+{
+    return e->wfd;
+}
+
 int event_notifier_set(EventNotifier *e)
 {
     static const uint64_t value = 1;
-- 
MST