[Qemu-devel] [PATCH] virtio_ring: use smp_store_mb

[Michael S. Tsirkin]

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

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 <address@hidden>
---

Seems to give a speedup on my box but I'm less sure about this one.  E.g. as
xchng faster than mfence on all/most intel CPUs? Anyone has an opinion?

 include/linux/virtio_ring.h  | 14 ++++++++++++++
 drivers/virtio/virtio_ring.c | 15 +++++++++------
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index 0135c16..8912189 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -47,6 +47,20 @@ static inline void virtio_wmb(bool weak_barriers)
                wmb();
 }
 
+static inline void virtio_store_mb(bool weak_barriers,
+                                  __virtio16 *p, __virtio16 v)
+{
+#ifdef CONFIG_SMP
+       if (weak_barriers)
+               smp_store_mb(*p, v);
+       else
+#endif
+       {
+               WRITE_ONCE(*p, v);
+               mb();
+       }
+}
+
 static inline __virtio16 virtio_load_acquire(bool weak_barriers, __virtio16 *p)
 {
        if (!weak_barriers) {
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index f822cab..b0aea67 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