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>

1 files changed, 5 insertions, 4 deletions

diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
index 157f6a26b939..42008395534d 100644
--- a/Documentation/lguest/lguest.c
+++ b/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,