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

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

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
+       {
+               WRITE_ONCE(*p, v);
+               mb();
+       }
+}
+
 struct virtio_device;
 struct virtqueue;
 
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index ee663c4..e12e385 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;
-- 
MST


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