virtio: Force use of power-of-two for descriptor ring sizes
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>
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index 5b88d21..1a4ed49 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -67,7 +67,7 @@
};
/* The standard layout for the ring is a continuous chunk of memory which looks
- * like this. The used fields will be aligned to a "num+1" boundary.
+ * like this. We assume num is a power of 2.
*
* struct vring
* {
@@ -79,8 +79,8 @@
* __u16 avail_idx;
* __u16 available[num];
*
- * // Padding so a correctly-chosen num value will cache-align used_idx.
- * char pad[sizeof(struct vring_desc) - sizeof(avail_flags)];
+ * // Padding to the next page boundary.
+ * char pad[];
*
* // A ring of used descriptor heads with free-running index.
* __u16 used_flags;
@@ -88,18 +88,21 @@
* struct vring_used_elem used[num];
* };
*/
-static inline void vring_init(struct vring *vr, unsigned int num, void *p)
+static inline void vring_init(struct vring *vr, unsigned int num, void *p,
+ unsigned int pagesize)
{
vr->num = num;
vr->desc = p;
vr->avail = p + num*sizeof(struct vring_desc);
- vr->used = p + (num+1)*(sizeof(struct vring_desc) + sizeof(__u16));
+ vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + pagesize-1)
+ & ~(pagesize - 1));
}
-static inline unsigned vring_size(unsigned int num)
+static inline unsigned vring_size(unsigned int num, unsigned int pagesize)
{
- return (num + 1) * (sizeof(struct vring_desc) + sizeof(__u16))
- + sizeof(__u32) + num * sizeof(struct vring_used_elem);
+ return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num)
+ + pagesize - 1) & ~(pagesize - 1))
+ + sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num;
}
#ifdef __KERNEL__