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virtio: cache used_idx in a VirtQueue field 

Accessing used_idx in the VQ requires an expensive access to
guest physical memory. Before this patch, 3 accesses are normally
done for each pop/push/notify call. However, since the used_idx is
only written by us, we can track it in our internal data structure.

Signed-off-by: Vincenzo Maffione <v.maffione@gmail.com>
Message-Id: <3d062ec54e9a7bf9fb325c1fd693564951f2b319.1450218353.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 11:29:04 +01:00 committed by Michael S. Tsirkin
parent aa570d6fb6
commit b796fcd1bf
1 changed files with 9 additions and 3 deletions
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12
hw/virtio/virtio.c
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@ -71,6 +71,9 @@ struct VirtQueue
{
VRing vring;
uint16_t last_avail_idx;
uint16_t used_idx;
/* Last used index value we have signalled on */
uint16_t signalled_used;
@ -170,6 +173,7 @@ static inline void vring_used_idx_set(VirtQueue *vq, uint16_t val)
hwaddr pa;
pa = vq->vring.used + offsetof(VRingUsed, idx);
virtio_stw_phys(vq->vdev, pa, val);
vq->used_idx = val;
}
static inline void vring_used_flags_set_bit(VirtQueue *vq, int mask)
@ -261,7 +265,7 @@ void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
virtqueue_unmap_sg(vq, elem, len);
idx = (idx + vring_used_idx(vq)) % vq->vring.num;
idx = (idx + vq->used_idx) % vq->vring.num;
/* Get a pointer to the next entry in the used ring. */
vring_used_ring_id(vq, idx, elem->index);
@ -274,7 +278,7 @@ void virtqueue_flush(VirtQueue *vq, unsigned int count)
/* Make sure buffer is written before we update index. */
smp_wmb();
trace_virtqueue_flush(vq, count);
old = vring_used_idx(vq);
old = vq->used_idx;
new = old + count;
vring_used_idx_set(vq, new);
vq->inuse -= count;
@ -782,6 +786,7 @@ 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].used_idx = 0;
virtio_queue_set_vector(vdev, i, VIRTIO_NO_VECTOR);
vdev->vq[i].signalled_used = 0;
vdev->vq[i].signalled_used_valid = false;
@ -1161,7 +1166,7 @@ static bool vring_notify(VirtIODevice *vdev, VirtQueue *vq)
v = vq->signalled_used_valid;
vq->signalled_used_valid = true;
old = vq->signalled_used;
new = vq->signalled_used = vring_used_idx(vq);
new = vq->signalled_used = vq->used_idx;
return !v || vring_need_event(vring_get_used_event(vq), new, old);
}
@ -1573,6 +1578,7 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)
vdev->vq[i].last_avail_idx, nheads);
return -1;
}
vdev->vq[i].used_idx = vring_used_idx(&vdev->vq[i]);
}
}