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 <m...@redhat.com>
Cc: <sta...@vger.kernel.org> # f277ec4 virtio_ring: shadow available
Cc: <sta...@vger.kernel.org>
Signed-off-by: Ladi Prosek <lpro...@redhat.com>
---
v1->v2:
* fixed coding style
* perf measurement results added to commit message
* patch sent to virtualization@lists.linux-foundation.org (was
k...@vger.kernel.org)
drivers/virtio/virtio_ring.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index e383ecd..926ecb7 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -732,7 +732,8 @@ 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);
}
}
@@ -764,7 +765,8 @@ 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);
@@ -832,10 +834,11 @@ 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;
@@ -953,7 +956,8 @@ 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. */
--
2.5.5
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