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/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
index 157f6a2..4200839 100644
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -62,8 +62,8 @@
#endif
/* We can have up to 256 pages for devices. */
#define DEVICE_PAGES 256
-/* This fits nicely in a single 4096-byte page. */
-#define VIRTQUEUE_NUM 127
+/* This will occupy 2 pages: it must be a power of 2. */
+#define VIRTQUEUE_NUM 128
/*L:120 verbose is both a global flag and a macro. The C preprocessor allows
* this, and although I wouldn't recommend it, it works quite nicely here. */
@@ -1036,7 +1036,8 @@
void *p;
/* First we need some pages for this virtqueue. */
- pages = (vring_size(num_descs) + getpagesize() - 1) / getpagesize();
+ pages = (vring_size(num_descs, getpagesize()) + getpagesize() - 1)
+ / getpagesize();
p = get_pages(pages);
/* Initialize the configuration. */
@@ -1045,7 +1046,7 @@
vq->config.pfn = to_guest_phys(p) / getpagesize();
/* Initialize the vring. */
- vring_init(&vq->vring, num_descs, p);
+ vring_init(&vq->vring, num_descs, p, getpagesize());
/* Add the configuration information to this device's descriptor. */
add_desc_field(dev, VIRTIO_CONFIG_F_VIRTQUEUE,
diff --git a/drivers/lguest/lguest_device.c b/drivers/lguest/lguest_device.c
index 8904f72..66f3872 100644
--- a/drivers/lguest/lguest_device.c
+++ b/drivers/lguest/lguest_device.c
@@ -200,7 +200,8 @@
/* Figure out how many pages the ring will take, and map that memory */
lvq->pages = lguest_map((unsigned long)lvq->config.pfn << PAGE_SHIFT,
- DIV_ROUND_UP(vring_size(lvq->config.num),
+ DIV_ROUND_UP(vring_size(lvq->config.num,
+ PAGE_SIZE),
PAGE_SIZE));
if (!lvq->pages) {
err = -ENOMEM;
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 0e1bf05..1dc04b6 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -277,11 +277,17 @@
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;
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__
