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authorRusty Russell <rusty@rustcorp.com.au>2007-11-12 05:39:18 +0300
committerRusty Russell <rusty@rustcorp.com.au>2007-11-12 05:59:40 +0300
commit42b36cc0ce717deeb10030141a43dede763a3ebe (patch)
treeb2dc48b4f16c5dc59461ad24b027d631edda1da4 /Documentation
parent1200e646ae238afc536be70257290eb33fb6e364 (diff)
downloadlinux-42b36cc0ce717deeb10030141a43dede763a3ebe.tar.xz
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>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/lguest/lguest.c9
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,