From 42b36cc0ce717deeb10030141a43dede763a3ebe Mon Sep 17 00:00:00 2001 From: Rusty Russell Date: Mon, 12 Nov 2007 13:39:18 +1100 Subject: 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 --- Documentation/lguest/lguest.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) (limited to 'Documentation/lguest') 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, -- cgit v1.2.1