[PATCH] virtio/ringtest: fix up need_event math

[PATCH] virtio/ringtest: fix up need_event math

[Michael S. Tsirkin]

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.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 tools/virtio/ringtest/ring.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/tools/virtio/ringtest/ring.c b/tools/virtio/ringtest/ring.c
index 6b70c4e..d044a88 100644
--- a/tools/virtio/ringtest/ring.c
+++ b/tools/virtio/ringtest/ring.c
@@ -196,16 +196,18 @@ bool enable_call()
 
 void kick_available(void)
 {
+	bool need;
+
 	/* Flush in previous flags write */
 	/* Barrier C (for pairing) */
 	smp_mb();
-	if (!need_event(event->kick_index,
-			guest.avail_idx,
-			guest.kicked_avail_idx))
-		return;
+	need = need_event(event->kick_index,
+			   guest.avail_idx,
+			   guest.kicked_avail_idx);
 
 	guest.kicked_avail_idx = guest.avail_idx;
-	kick();
+	if (need)
+		kick();
 }
 
 /* host side */
@@ -265,14 +267,18 @@ 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 (!need_event(event->call_index,
+
+	need = !need_event(event->call_index,
 			host.used_idx,
-			host.called_used_idx))
-		return;
+			host.called_used_idx);
 
 	host.called_used_idx = host.used_idx;
-	call();
+
+	if (need)
+		call();
 }
-- 
MST

Re: [PATCH] virtio/ringtest: fix up need_event math

[Cornelia Huck]

On Thu, 26 Oct 2017 04:48:01 +0300
"Michael S. Tsirkin" <mst@redhat.com> 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.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  tools/virtio/ringtest/ring.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)

Acked-by: Cornelia Huck <cohuck@redhat.com>

I think virtio_ring_0_9 has the same issue?

Re: [PATCH] virtio/ringtest: fix up need_event math

[Michael S. Tsirkin]

On Thu, Oct 26, 2017 at 10:36:47AM +0200, Cornelia Huck wrote:
> On Thu, 26 Oct 2017 04:48:01 +0300
> "Michael S. Tsirkin" <mst@redhat.com> 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.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  tools/virtio/ringtest/ring.c | 24 +++++++++++++++---------
> >  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> Acked-by: Cornelia Huck <cohuck@redhat.com>

Actually it seems to sometimes trigger stalls when host and guest
are on the same physical core. To trigger:

./ring --sleep --host-affinity 0 --guest-affinity 0

This worries me since it's similar to what vhost does.

Still debugging - anyone see anything suspicious?


> I think virtio_ring_0_9 has the same issue?

I think it does, posted a patch.

-- 
MST

Re: [PATCH] virtio/ringtest: fix up need_event math

[Jason Wang]

On 2017年10月26日 09:48, 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.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>   tools/virtio/ringtest/ring.c | 24 +++++++++++++++---------
>   1 file changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/tools/virtio/ringtest/ring.c b/tools/virtio/ringtest/ring.c
> index 6b70c4e..d044a88 100644
> --- a/tools/virtio/ringtest/ring.c
> +++ b/tools/virtio/ringtest/ring.c
> @@ -196,16 +196,18 @@ bool enable_call()
>   
>   void kick_available(void)
>   {
> +	bool need;
> +
>   	/* Flush in previous flags write */
>   	/* Barrier C (for pairing) */
>   	smp_mb();
> -	if (!need_event(event->kick_index,
> -			guest.avail_idx,
> -			guest.kicked_avail_idx))
> -		return;
> +	need = need_event(event->kick_index,
> +			   guest.avail_idx,
> +			   guest.kicked_avail_idx);
>   
>   	guest.kicked_avail_idx = guest.avail_idx;
> -	kick();
> +	if (need)
> +		kick();
>   }
>   
>   /* host side */
> @@ -265,14 +267,18 @@ 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 (!need_event(event->call_index,
> +
> +	need = !need_event(event->call_index,
>   			host.used_idx,
> -			host.called_used_idx))
> -		return;
> +			host.called_used_idx);
>   
>   	host.called_used_idx = host.used_idx;
> -	call();
> +
> +	if (need)
> +		call();
>   }

Acked-by: Jason Wang <jasowang@redhat.com>