Linux Virtualization - Jan 2012 - [PATCH 1/2] virtio: fix typos of memory barriers

Signed-off-by: Jason Wang <jasowang at redhat.com>
---
 drivers/virtio/virtio_ring.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 79e1b29..78428a8 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -35,7 +35,7 @@
 #define virtio_rmb(vq) \
 	do { if ((vq)->weak_barriers) smp_rmb(); else rmb(); } while(0)
 #define virtio_wmb(vq) \
-	do { if ((vq)->weak_barriers) smp_rmb(); else rmb(); } while(0)
+	do { if ((vq)->weak_barriers) smp_wmb(); else wmb(); } while(0)
 #else
 /* We must force memory ordering even if guest is UP since host could be
  * running on another CPU, but SMP barriers are defined to barrier() in that

Use virtio_mb() to make sure the available index to be exposed before
checking the the avail event. Otherwise we may get stale value of
avail event in guest and never kick the host after.

Signed-off-by: Jason Wang <jasowang at redhat.com>
---
 drivers/virtio/virtio_ring.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 78428a8..07d9360 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -308,9 +308,9 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
 	bool needs_kick;
 
 	START_USE(vq);
-	/* Descriptors and available array need to be set before we expose the
-	 * new available array entries. */
-	virtio_wmb(vq);
+	/* We need expose available array entries before checking avail
+	 * event. */
+	virtio_mb(vq);
 
 	old = vq->vring.avail->idx - vq->num_added;
 	new = vq->vring.avail->idx;

On Fri, Jan 20, 2012 at 04:16:59PM +0800, Jason Wang
wrote:
> Signed-off-by: Jason Wang <jasowang at redhat.com>
Good catch.
Note: this fixes a bug introduced by
7b21e34fd1c272e3a8c3846168f2f6287a4cd72b.
It's probably a good idea to mention
this is the commit log.

Acked-by: Michael S. Tsirkin <mst at redhat.com>


> ---
>  drivers/virtio/virtio_ring.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 79e1b29..78428a8 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -35,7 +35,7 @@
>  #define virtio_rmb(vq) \
>  	do { if ((vq)->weak_barriers) smp_rmb(); else rmb(); } while(0)
>  #define virtio_wmb(vq) \
> -	do { if ((vq)->weak_barriers) smp_rmb(); else rmb(); } while(0)
> +	do { if ((vq)->weak_barriers) smp_wmb(); else wmb(); } while(0)
>  #else
>  /* We must force memory ordering even if guest is UP since host could be
>   * running on another CPU, but SMP barriers are defined to barrier() in
that

On Fri, Jan 20, 2012 at 04:17:08PM +0800, Jason Wang
wrote:
> Use virtio_mb() to make sure the available index to be exposed before
> checking the the avail event. Otherwise we may get stale value of
> avail event in guest and never kick the host after.
> 
> Signed-off-by: Jason Wang <jasowang at redhat.com>
Good catch.
Note: this fixes a bug introduced by ee7cd8981e15bcb365fc762afe3fc47b8242f630.

Acked-by: Michael S. Tsirkin <mst at redhat.com>
> ---
>  drivers/virtio/virtio_ring.c |    6 +++---
>  1 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 78428a8..07d9360 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -308,9 +308,9 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
>  	bool needs_kick;
>  
>  	START_USE(vq);
> -	/* Descriptors and available array need to be set before we expose the
> -	 * new available array entries. */
> -	virtio_wmb(vq);
> +	/* We need expose available array entries before checking avail
Nit:
We need expose -> Need to expose
> +	 * event. */
> +	virtio_mb(vq);
>  
>  	old = vq->vring.avail->idx - vq->num_added;
>  	new = vq->vring.avail->idx;