virtio_ring: Make interrupt suppression spec compliant

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 <[e-mail]>
Cc: <[e-mail]> # f277ec4 virtio_ring: shadow available
Cc: <[e-mail]>
Signed-off-by: Ladi Prosek <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/virtio/virtio_ring.c

···
 
 	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);
 	}
 
 }
···
 	 * 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);
···
 	 * 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;
···
 	/* 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. */