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virtio: Force use of power-of-two for descriptor ring sizes

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The virtio descriptor rings of size N-1 were nicely set up to be
aligned to an N-byte boundary.  But as Anthony Liguori points out, the
free-running indices used by virtio require that the sizes be a power
of 2, otherwise we get problems on wrap (demonstrated with lguest).

So we replace the clever "2^n-1" scheme with a simple "align to page
boundary" scheme: this means that all virtio rings take at least two
pages, but it's safer than guessing cache alignment.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Rusty Russell
2007-11-12 13:39:18 +11:00
parent 1200e646ae
commit 42b36cc0ce
4 changed files with 25 additions and 14 deletions
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8
drivers/virtio/virtio_ring.c
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@@ -277,11 +277,17 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
struct vring_virtqueue *vq;
unsigned int i;
/* We assume num is a power of 2. */
if (num & (num - 1)) {
dev_warn(&vdev->dev, "Bad virtqueue length %u\n", num);
return NULL;
}
vq = kmalloc(sizeof(*vq) + sizeof(void *)*num, GFP_KERNEL);
if (!vq)
return NULL;
vring_init(&vq->vring, num, pages);
vring_init(&vq->vring, num, pages, PAGE_SIZE);
vq->vq.callback = callback;
vq->vq.vdev = vdev;
vq->vq.vq_ops = &vring_vq_ops;