From 42b36cc0ce717deeb10030141a43dede763a3ebe Mon Sep 17 00:00:00 2001 From: Rusty Russell Date: Mon, 12 Nov 2007 13:39:18 +1100 Subject: 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 --- include/linux/virtio_ring.h | 19 +++++++++++-------- 1 file changed, 11 insertions(+), 8 deletions(-) (limited to 'include/linux') diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h index 5b88d215af50..1a4ed49f6478 100644 --- a/include/linux/virtio_ring.h +++ b/include/linux/virtio_ring.h @@ -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__ -- cgit v1.2.3-59-g8ed1b