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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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19
include/linux/virtio_ring.h
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@ -67,7 +67,7 @@ struct vring {
};
/* 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 @@ struct vring {
* __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 {
* 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__