Linux Virtualization - Oct 2017 - [PATCH] virtio/ringtest: virtio_ring: fix up need_event math

last kicked event index must be updated unconditionally:
even if we don't need to kick, we do not want to re-check
the same entry for events.

Reported-by: Cornelia Huck <cohuck at redhat.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---
 tools/virtio/ringtest/virtio_ring_0_9.c | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/tools/virtio/ringtest/virtio_ring_0_9.c
b/tools/virtio/ringtest/virtio_ring_0_9.c
index bbc3043..5fd3fbc 100644
--- a/tools/virtio/ringtest/virtio_ring_0_9.c
+++ b/tools/virtio/ringtest/virtio_ring_0_9.c
@@ -225,16 +225,18 @@ bool enable_call()
 
 void kick_available(void)
 {
+	bool need;
+
 	/* Flush in previous flags write */
 	/* Barrier C (for pairing) */
 	smp_mb();
-	if (!vring_need_event(vring_avail_event(&ring),
-			      guest.avail_idx,
-			      guest.kicked_avail_idx))
-		return;
+	need = vring_need_event(vring_avail_event(&ring),
+				guest.avail_idx,
+				guest.kicked_avail_idx);
 
 	guest.kicked_avail_idx = guest.avail_idx;
-	kick();
+	if (need)
+		kick();
 }
 
 /* host side */
@@ -316,14 +318,16 @@ bool use_buf(unsigned *lenp, void **bufp)
 
 void call_used(void)
 {
+	bool need;
+
 	/* Flush in previous flags write */
 	/* Barrier D (for pairing) */
 	smp_mb();
-	if (!vring_need_event(vring_used_event(&ring),
-			      host.used_idx,
-			      host.called_used_idx))
-		return;
+	need = vring_need_event(vring_used_event(&ring),
+				host.used_idx,
+				host.called_used_idx);
 
 	host.called_used_idx = host.used_idx;
-	call();
+	if (need)
+		call();
 }
-- 
MST

On Fri, Oct 27, 2017 at 01:11:13PM +0000, Michael S. Tsirkin
wrote:
>last kicked event index must be updated unconditionally:
>even if we don't need to kick, we do not want to re-check
>the same entry for events.
>
>Reported-by: Cornelia Huck <cohuck at redhat.com>
>Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
>---
> tools/virtio/ringtest/virtio_ring_0_9.c | 24 ++++++++++++++----------
> 1 file changed, 14 insertions(+), 10 deletions(-)
>
Reviewed-by: Jens Freimann <jfreimann at redhat.com>