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virtio: Force use of power-of-two for descriptor ring sizes

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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 <[email protected]>
This commit is contained in:
Rusty Russell 2007-11-12 13:39:18 +11:00
commit 42b36cc0ce
4 changed files with 25 additions and 14 deletions
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9
Documentation/lguest/lguest.c
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@ -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,