Linux Virtualization - Nov 2011 - [PATCH 4 of 5] virtio: avoid modulus operation

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 at 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);

On Thu, Nov 3, 2011 at 9:42 AM, Rusty Russell <rusty at rustcorp.com.au>
wrote:
> 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 at 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);
What's this hunk here?

On Thu, 3 Nov 2011 09:51:15 +0200, Pekka Enberg <penberg at kernel.org>
wrote:
> On Thu, Nov 3, 2011 at 9:42 AM, Rusty Russell <rusty at
rustcorp.com.au> wrote:
> > 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 at 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);
> 
> What's this hunk here?
Ugly workaround for console until hvc_console breakage gets reverted.

Left in by mistake :)

Thanks,
Rusty.