virtio_ring: Make interrupt suppression spec compliant

BugLink: http://bugs.launchpad.net/bugs/1642572
commit 0ea1e4a6d9b62cf29e210d2b4ba9fd43917522e3 upstream.

According to the spec, if the VIRTIO_RING_F_EVENT_IDX feature bit is
negotiated the driver MUST set flags to 0. Not dirtying the available
ring in virtqueue_disable_cb also has a minor positive performance
impact, improving L1 dcache load missed by ~0.5% in vring_bench.

Writes to the used event field (vring_used_event) are still unconditional.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index dc2b94142f53251560e0734b769f5bf5f7f1feb0..a01a41a4126934c4b9af04853e409011aec3bbce 100644 (file)
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -548,7 +548,8 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
 
        if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
                vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
-               vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
+               if (!vq->event)
+                       vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
        }
 
 }
@@ -580,7 +581,8 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
         * entry. Always do both to keep code simple. */
        if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
                vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
-               vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
+               if (!vq->event)
+                       vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
        }
        vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, last_used_idx = vq->last_used_idx);
        END_USE(vq);
@@ -648,10 +650,11 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
         * more to do. */
        /* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
         * either clear the flags bit or point the event index at the next
-        * entry. Always do both to keep code simple. */
+        * entry. Always update the event index to keep code simple. */
        if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
                vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
-               vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
+               if (!vq->event)
+                       vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
        }
        /* TODO: tune this threshold */
        bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4;
@@ -770,7 +773,8 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
        /* No callback?  Tell other side not to bother us. */
        if (!callback) {
                vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
-               vq->vring.avail->flags = cpu_to_virtio16(vdev, vq->avail_flags_shadow);
+               if (!vq->event)
+                       vq->vring.avail->flags = cpu_to_virtio16(vdev, vq->avail_flags_shadow);
        }
 
        /* Put everything in free lists. */