virtio_ring: use virt_store_mb

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@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

2 files changed, 20 insertions, 6 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index ee663c458b20..e12e385f7ac3 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -517,10 +517,10 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
         /* If we expect an interrupt for the next entry, tell host
          * by writing event index and flush out the write before
          * the read in the next get_buf call. */
-        if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
-                vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, vq->last_used_idx);
-                virtio_mb(vq->weak_barriers);
-        }
+        if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT))
+                virtio_store_mb(vq->weak_barriers,
+                                &vring_used_event(&vq->vring),
+                                cpu_to_virtio16(_vq->vdev, vq->last_used_idx));
 
 #ifdef DEBUG
         vq->last_add_time_valid = false;
@@ -653,8 +653,11 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
         }
         /* TODO: tune this threshold */
         bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4;
-        vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs);
-        virtio_mb(vq->weak_barriers);
+
+        virtio_store_mb(vq->weak_barriers,
+                        &vring_used_event(&vq->vring),
+                        cpu_to_virtio16(_vq->vdev, vq->last_used_idx + bufs));
+
         if (unlikely((u16)(virtio16_to_cpu(_vq->vdev, vq->vring.used->idx) - vq->last_used_idx) > bufs)) {
                 END_USE(vq);
                 return false;
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index f3fa55bdd6ce..a156e2b6ccfe 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -45,6 +45,17 @@ 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 {
+                WRITE_ONCE(*p, v);
+                mb();
+        }
+}
+
 struct virtio_device;
 struct virtqueue;