| Message ID | 1448874894-18398-1-git-send-email-mst@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 11/30/2015 10:15 AM, Michael S. Tsirkin wrote: > We know vring num is a power of 2, so use & > to mask the high bits. Makes a lot of sense and virtio_ring.c seems to use the same logic. Acked-by: Christian Borntraeger <borntraeger@de.ibm.com> > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > --- > > Changes from v1: drop an unrelated chunk > > drivers/vhost/vhost.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index 080422f..ad2146a 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -1369,7 +1369,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq, > /* Grab the next descriptor number they're advertising, and increment > * the index we've seen. */ > if (unlikely(__get_user(ring_head, > - &vq->avail->ring[last_avail_idx % vq->num]))) { > + &vq->avail->ring[last_avail_idx & (vq->num - 1)]))) { > vq_err(vq, "Failed to read head: idx %d address %p\n", > last_avail_idx, > &vq->avail->ring[last_avail_idx % vq->num]); > @@ -1489,7 +1489,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq, > u16 old, new; > int start; > > - start = vq->last_used_idx % vq->num; > + start = vq->last_used_idx & (vq->num - 1); > used = vq->used->ring + start; > if (count == 1) { > if (__put_user(heads[0].id, &used->id)) { > @@ -1531,7 +1531,7 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads, > { > int start, n, r; > > - start = vq->last_used_idx % vq->num; > + start = vq->last_used_idx & (vq->num - 1); > n = vq->num - start; > if (n < count) { > r = __vhost_add_used_n(vq, heads, n); > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
From: "Michael S. Tsirkin" <mst@redhat.com> Date: Mon, 30 Nov 2015 11:15:23 +0200 > We know vring num is a power of 2, so use & > to mask the high bits. > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: David S. Miller <davem@davemloft.net> -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Mon, Nov 30, 2015 at 11:15:23AM +0200, Michael S. Tsirkin wrote: > We know vring num is a power of 2, so use & > to mask the high bits. > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > --- The generated code switches from DIV -> masking, source is clearer as well. Tested-by: Venkatesh Srinivas <venkateshs@google.com> -- vs; -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, Dec 4, 2015 at 10:19 PM, Venkatesh Srinivas <venkateshs@google.com> wrote: > On Mon, Nov 30, 2015 at 11:15:23AM +0200, Michael S. Tsirkin wrote: >> We know vring num is a power of 2, so use & >> to mask the high bits. >> >> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> >> --- > > The generated code switches from DIV -> masking, source is clearer as well. First impression was why, now it seems that compiler can't predict this for variables. For constants it would be optimized to the same, I suppose. > > Tested-by: Venkatesh Srinivas <venkateshs@google.com> > > -- vs; > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index 080422f..ad2146a 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -1369,7 +1369,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq, /* Grab the next descriptor number they're advertising, and increment * the index we've seen. */ if (unlikely(__get_user(ring_head, - &vq->avail->ring[last_avail_idx % vq->num]))) { + &vq->avail->ring[last_avail_idx & (vq->num - 1)]))) { vq_err(vq, "Failed to read head: idx %d address %p\n", last_avail_idx, &vq->avail->ring[last_avail_idx % vq->num]); @@ -1489,7 +1489,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq, u16 old, new; int start; - start = vq->last_used_idx % vq->num; + start = vq->last_used_idx & (vq->num - 1); used = vq->used->ring + start; if (count == 1) { if (__put_user(heads[0].id, &used->id)) { @@ -1531,7 +1531,7 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads, { int start, n, r; - start = vq->last_used_idx % vq->num; + start = vq->last_used_idx & (vq->num - 1); n = vq->num - start; if (n < count) { r = __vhost_add_used_n(vq, heads, n);
We know vring num is a power of 2, so use & to mask the high bits. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> --- Changes from v1: drop an unrelated chunk drivers/vhost/vhost.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)