Commit a5c262c5 authored by Michael S. Tsirkin's avatar Michael S. Tsirkin Committed by Rusty Russell
Browse files

virtio_ring: support event idx feature 



Support for the new event idx feature:
1. When enabling interrupts, publish the current avail index
   value to the host to get interrupts on the next update.
2. Use the new avail_event feature to reduce the number
   of exits from the guest.

Simple test with the simulator:

[virtio]# time ./virtio_test
spurious wakeus: 0x7

real    0m0.169s
user    0m0.140s
sys     0m0.019s
[virtio]# time ./virtio_test --no-event-idx
spurious wakeus: 0x11

real    0m0.649s
user    0m0.295s
sys     0m0.335s

Signed-off-by: default avatarMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: default avatarRusty Russell <rusty@rustcorp.com.au>
parent bf7035bf

drivers/virtio/virtio_ring.c

+24 −2
Original line number Diff line number Diff line
@@ -82,6 +82,9 @@ struct vring_virtqueue 
	/* Host supports indirect buffers */

	bool indirect;



	/* Host publishes avail event idx */

	bool event;



	/* Number of free buffers */

	unsigned int num_free;

	/* Head of free buffer list. */
@@ -237,18 +240,22 @@ EXPORT_SYMBOL_GPL(virtqueue_add_buf_gfp); 
void virtqueue_kick(struct virtqueue *_vq)

{

	struct vring_virtqueue *vq = to_vvq(_vq);

	u16 new, old;

	START_USE(vq);

	/* Descriptors and available array need to be set before we expose the

	 * new available array entries. */

	virtio_wmb();



	vq->vring.avail->idx += vq->num_added;

	old = vq->vring.avail->idx;

	new = vq->vring.avail->idx = old + vq->num_added;

	vq->num_added = 0;



	/* Need to update avail index before checking if we should notify */

	virtio_mb();



	if (!(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY))

	if (vq->event ?

	    vring_need_event(vring_avail_event(&vq->vring), new, old) :

	    !(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY))

		/* Prod other side to tell it about changes. */

		vq->notify(&vq->vq);
@@ -324,6 +331,14 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len) 
	ret = vq->data[i];

	detach_buf(vq, i);

	vq->last_used_idx++;

	/* 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->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)) {

		vring_used_event(&vq->vring) = vq->last_used_idx;

		virtio_mb();

	}



	END_USE(vq);

	return ret;

}
@@ -345,7 +360,11 @@ bool virtqueue_enable_cb(struct virtqueue *_vq) 


	/* We optimistically turn back on interrupts, then check if there was

	 * more to do. */

	/* Depending on the VIRTIO_RING_F_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. */

	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;

	vring_used_event(&vq->vring) = vq->last_used_idx;

	virtio_mb();

	if (unlikely(more_used(vq))) {

		END_USE(vq);
@@ -438,6 +457,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int num, 
#endif



	vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);

	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);



	/* No callback?  Tell other side not to bother us. */

	if (!callback)
@@ -472,6 +492,8 @@ void vring_transport_features(struct virtio_device *vdev) 
		switch (i) {

		case VIRTIO_RING_F_INDIRECT_DESC:

			break;

		case VIRTIO_RING_F_EVENT_IDX:

			break;

		default:

			/* We don't understand this bit. */

			clear_bit(i, vdev->features);

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