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>

Documentation/lguest/lguest.c

@@ -62,8 +62,8 @@ typedef uint8_t u8;
 #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 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs,
 	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 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs,
 	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,

drivers/lguest/lguest_device.c

@@ -200,7 +200,8 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 
 	/* 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;

drivers/virtio/virtio_ring.c

@@ -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;

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__