Commit 0ea1e4a6 authored by Ladi Prosek's avatar Ladi Prosek Committed by Michael S. Tsirkin
Browse files

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 <mst@redhat.com>
Cc: <stable@vger.kernel.org> # f277ec42 virtio_ring: shadow available
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarLadi Prosek <lprosek@redhat.com>
Signed-off-by: default avatarMichael S. Tsirkin <mst@redhat.com>
parent a0be1db4

drivers/virtio/virtio_ring.c

+9 −5
Original line number Diff line number Diff line
@@ -732,6 +732,7 @@ 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;

		if (!vq->event)

			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);

	}
@@ -764,6 +765,7 @@ 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;

		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);
@@ -832,9 +834,10 @@ 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;

		if (!vq->event)

			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);

	}

	/* TODO: tune this threshold */
@@ -953,6 +956,7 @@ 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;

		if (!vq->event)

			vq->vring.avail->flags = cpu_to_virtio16(vdev, vq->avail_flags_shadow);

	}

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