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virtio: read avail_idx from VQ only when necessary 

The virtqueue_pop() implementation needs to check if the avail ring
contains some pending buffers. To perform this check, it is not
always necessary to fetch the avail_idx in the VQ memory, which is
expensive. This patch introduces a shadow variable tracking avail_idx
and modifies virtio_queue_empty() to access avail_idx in physical
memory only when necessary.

Signed-off-by: Vincenzo Maffione <v.maffione@gmail.com>
Message-Id: <b617d6459902773d9f4ab843bfaca764f5af8eda.1450218353.git.v.maffione@gmail.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Vincenzo Maffione 2016-01-31 11:29:05 +01:00 committed by Michael S. Tsirkin
parent b796fcd1bf
commit be1fea9bc2
1 changed files with 22 additions and 4 deletions
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26
hw/virtio/virtio.c
View File

@ -70,8 +70,13 @@ typedef struct VRing
struct VirtQueue
{
VRing vring;
/* Next head to pop */
uint16_t last_avail_idx;
/* Last avail_idx read from VQ. */
uint16_t shadow_avail_idx;
uint16_t used_idx;
/* Last used index value we have signalled on */
@ -132,7 +137,8 @@ static inline uint16_t vring_avail_idx(VirtQueue *vq)
{
hwaddr pa;
pa = vq->vring.avail + offsetof(VRingAvail, idx);
return virtio_lduw_phys(vq->vdev, pa);
vq->shadow_avail_idx = virtio_lduw_phys(vq->vdev, pa);
return vq->shadow_avail_idx;
}
static inline uint16_t vring_avail_ring(VirtQueue *vq, int i)
@ -223,8 +229,14 @@ int virtio_queue_ready(VirtQueue *vq)
return vq->vring.avail != 0;
}
/* Fetch avail_idx from VQ memory only when we really need to know if
* guest has added some buffers. */
int virtio_queue_empty(VirtQueue *vq)
{
if (vq->shadow_avail_idx != vq->last_avail_idx) {
return 0;
}
return vring_avail_idx(vq) == vq->last_avail_idx;
}
@ -300,7 +312,7 @@ static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx)
/* Check it isn't doing very strange things with descriptor numbers. */
if (num_heads > vq->vring.num) {
error_report("Guest moved used index from %u to %u",
idx, vring_avail_idx(vq));
idx, vq->shadow_avail_idx);
exit(1);
}
/* On success, callers read a descriptor at vq->last_avail_idx.
@ -535,9 +547,12 @@ void *virtqueue_pop(VirtQueue *vq, size_t sz)
struct iovec iov[VIRTQUEUE_MAX_SIZE];
VRingDesc desc;
if (!virtqueue_num_heads(vq, vq->last_avail_idx)) {
if (virtio_queue_empty(vq)) {
return NULL;
}
/* Needed after virtio_queue_empty(), see comment in
* virtqueue_num_heads(). */
smp_rmb();
/* When we start there are none of either input nor output. */
out_num = in_num = 0;
@ -786,6 +801,7 @@ void virtio_reset(void *opaque)
vdev->vq[i].vring.avail = 0;
vdev->vq[i].vring.used = 0;
vdev->vq[i].last_avail_idx = 0;
vdev->vq[i].shadow_avail_idx = 0;
vdev->vq[i].used_idx = 0;
virtio_queue_set_vector(vdev, i, VIRTIO_NO_VECTOR);
vdev->vq[i].signalled_used = 0;
@ -1155,7 +1171,7 @@ static bool vring_notify(VirtIODevice *vdev, VirtQueue *vq)
smp_mb();
/* Always notify when queue is empty (when feature acknowledge) */
if (virtio_vdev_has_feature(vdev, VIRTIO_F_NOTIFY_ON_EMPTY) &&
!vq->inuse && vring_avail_idx(vq) == vq->last_avail_idx) {
!vq->inuse && virtio_queue_empty(vq)) {
return true;
}
@ -1579,6 +1595,7 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
return -1;
}
vdev->vq[i].used_idx = vring_used_idx(&vdev->vq[i]);
vdev->vq[i].shadow_avail_idx = vring_avail_idx(&vdev->vq[i]);
}
}
@ -1714,6 +1731,7 @@ uint16_t virtio_queue_get_last_avail_idx(VirtIODevice *vdev, int n)
void virtio_queue_set_last_avail_idx(VirtIODevice *vdev, int n, uint16_t idx)
{
vdev->vq[n].last_avail_idx = idx;
vdev->vq[n].shadow_avail_idx = idx;
}
void virtio_queue_invalidate_signalled_used(VirtIODevice *vdev, int n)