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Re: [Xen-devel] [PATCH v2 32/32] virtio_ring: use virt_store_mb



Hello.

On 12/31/2015 10:09 PM, Michael S. Tsirkin wrote:

We need a full barrier after writing out event index, using
virt_store_mb there seems better than open-coding.  As usual, we need a
wrapper to account for strong barriers.

It's tempting to use this in vhost as well, for that, we'll
need a variant of smp_store_mb that works on __user pointers.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---
  include/linux/virtio_ring.h  | 12 ++++++++++++
  drivers/virtio/virtio_ring.c | 15 +++++++++------
  2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index f3fa55b..3a74d91 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -45,6 +45,18 @@ static inline void virtio_wmb(bool weak_barriers)
                wmb();
  }

+static inline void virtio_store_mb(bool weak_barriers,
+                                  __virtio16 *p, __virtio16 v)
+{
+       if (weak_barriers)
+               virt_store_mb(*p, v);
+       else
+       {

   The kernel coding style dictates:

        if (weak_barriers) {
                virt_store_mb(*p, v);
        } else {

+               WRITE_ONCE(*p, v);
+               mb();
+       }
+}
+
[...]

MBR, Sergei


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