diff options
| author | Rusty Russell <rusty@rustcorp.com.au> | 2007-11-12 03:39:18 +0100 |
|---|---|---|
| committer | Rusty Russell <rusty@rustcorp.com.au> | 2007-11-12 03:59:40 +0100 |
| commit | 42b36cc0ce717deeb10030141a43dede763a3ebe (patch) | |
| tree | b2dc48b4f16c5dc59461ad24b027d631edda1da4 /Documentation | |
| parent | lguest: Fix lguest virtio-blk backend size computation (diff) | |
| download | linux-42b36cc0ce717deeb10030141a43dede763a3ebe.tar.xz linux-42b36cc0ce717deeb10030141a43dede763a3ebe.zip | |
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.c | 9 |
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, |