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virtio: support unlocked queue poll

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This adds a way to check ring empty state after enable_cb outside any
locks. Will be used by virtio_net.

Note: there's room for more optimization: caller is likely to have a
memory barrier already, which means we might be able to get rid of a
barrier here.  Deferring this optimization until we do some
benchmarking.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Michael S. Tsirkin 2013-07-09 13:19:18 +03:00 committed by David S. Miller
parent 01276ed242
commit cc229884d3
2 changed files with 55 additions and 19 deletions
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70
drivers/virtio/virtio_ring.c
View file

@ -606,6 +606,55 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
}
EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
/**
* virtqueue_enable_cb_prepare - restart callbacks after disable_cb
* @vq: the struct virtqueue we're talking about.
*
* This re-enables callbacks; it returns current queue state
* in an opaque unsigned value. This value should be later tested by
* virtqueue_poll, to detect a possible race between the driver checking for
* more work, and enabling callbacks.
*
* Caller must ensure we don't call this with other virtqueue
* operations at the same time (except where noted).
*/
unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
{
struct vring_virtqueue *vq = to_vvq(_vq);
u16 last_used_idx;
START_USE(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) = last_used_idx = vq->last_used_idx;
END_USE(vq);
return last_used_idx;
}
EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
/**
* virtqueue_poll - query pending used buffers
* @vq: the struct virtqueue we're talking about.
* @last_used_idx: virtqueue state (from call to virtqueue_enable_cb_prepare).
*
* Returns "true" if there are pending used buffers in the queue.
*
* This does not need to be serialized.
*/
bool virtqueue_poll(struct virtqueue *_vq, unsigned last_used_idx)
{
struct vring_virtqueue *vq = to_vvq(_vq);
virtio_mb(vq->weak_barriers);
return (u16)last_used_idx != vq->vring.used->idx;
}
EXPORT_SYMBOL_GPL(virtqueue_poll);
/**
* virtqueue_enable_cb - restart callbacks after disable_cb.
* @vq: the struct virtqueue we're talking about.
@ -619,25 +668,8 @@ EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
*/
bool virtqueue_enable_cb(struct virtqueue *_vq)
{
struct vring_virtqueue *vq = to_vvq(_vq);
START_USE(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(vq->weak_barriers);
if (unlikely(more_used(vq))) {
END_USE(vq);
return false;
}
END_USE(vq);
return true;
unsigned last_used_idx = virtqueue_enable_cb_prepare(_vq);
return !virtqueue_poll(_vq, last_used_idx);
}
EXPORT_SYMBOL_GPL(virtqueue_enable_cb);

4
include/linux/virtio.h
View file

@ -70,6 +70,10 @@ void virtqueue_disable_cb(struct virtqueue *vq);
bool virtqueue_enable_cb(struct virtqueue *vq);
unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
bool virtqueue_poll(struct virtqueue *vq, unsigned);
bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
void *virtqueue_detach_unused_buf(struct virtqueue *vq);