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Subject[PATCH 4 of 5] virtio: avoid modulus operation
Date
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Since we know vq->vring.num is a power of 2, modulus is lazy (it's asserted
in vring_new_virtqueue()).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
drivers/virtio/virtio_ring.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -1420,7 +1420,7 @@ __init void lguest_init(void)
add_preferred_console("hvc", 0, NULL);

/* Register our very early console. */
- virtio_cons_early_init(early_put_chars);
+// virtio_cons_early_init(early_put_chars);

/*
* Last of all, we set the power management poweroff hook to point to
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -226,8 +226,8 @@ add_head:
vq->data[head] = data;

/* Put entry in available array (but don't update avail->idx until they
- * do sync). FIXME: avoid modulus here? */
- avail = (vq->vring.avail->idx + vq->num_added++) % vq->vring.num;
+ * do sync). */
+ avail = ((vq->vring.avail->idx + vq->num_added++) & (vq->vring.num-1));
vq->vring.avail->ring[avail] = head;

pr_debug("Added buffer head %i to %p\n", head, vq);
@@ -317,6 +317,7 @@ void *virtqueue_get_buf(struct virtqueue
struct vring_virtqueue *vq = to_vvq(_vq);
void *ret;
unsigned int i;
+ u16 last_used;

START_USE(vq);

@@ -334,8 +335,9 @@ void *virtqueue_get_buf(struct virtqueue
/* Only get used array entries after they have been exposed by host. */
virtio_rmb();

- i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
- *len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;
+ last_used = (vq->last_used_idx & (vq->vring.num - 1));
+ i = vq->vring.used->ring[last_used].id;
+ *len = vq->vring.used->ring[last_used].len;

if (unlikely(i >= vq->vring.num)) {
BAD_RING(vq, "id %u out of range\n", i);



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