Commit 42b36cc0 authored by Rusty Russell's avatar Rusty Russell
Browse files

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: default avatarRusty Russell <rusty@rustcorp.com.au>
parent 1200e646

Documentation/lguest/lguest.c

+5 −4
Original line number Diff line number Diff line
@@ -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,

drivers/lguest/lguest_device.c

+2 −1
Original line number Diff line number Diff line
@@ -200,7 +200,8 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev, 


	/* Figure out how many pages the ring will take, and map that memory */

	lvq->pages = lguest_map((unsigned long)lvq->config.pfn << PAGE_SHIFT,

				DIV_ROUND_UP(vring_size(lvq->config.num),

				DIV_ROUND_UP(vring_size(lvq->config.num,

							PAGE_SIZE),

					     PAGE_SIZE));

	if (!lvq->pages) {

		err = -ENOMEM;

drivers/virtio/virtio_ring.c

+7 −1
Original line number Diff line number Diff line
@@ -277,11 +277,17 @@ struct virtqueue *vring_new_virtqueue(unsigned int num, 
	struct vring_virtqueue *vq;

	unsigned int i;



	/* We assume num is a power of 2. */

	if (num & (num - 1)) {

		dev_warn(&vdev->dev, "Bad virtqueue length %u\n", num);

		return NULL;

	}



	vq = kmalloc(sizeof(*vq) + sizeof(void *)*num, GFP_KERNEL);

	if (!vq)

		return NULL;



	vring_init(&vq->vring, num, pages);

	vring_init(&vq->vring, num, pages, PAGE_SIZE);

	vq->vq.callback = callback;

	vq->vq.vdev = vdev;

	vq->vq.vq_ops = &vring_vq_ops;

include/linux/virtio_ring.h

+11 −8
Original line number Diff line number Diff line
@@ -67,7 +67,7 @@ struct vring { 
};



/* The standard layout for the ring is a continuous chunk of memory which looks

 * like this.  The used fields will be aligned to a "num+1" boundary.

 * like this.  We assume num is a power of 2.

 *

 * struct vring

 * {
@@ -79,8 +79,8 @@ struct vring { 
 *	__u16 avail_idx;

 *	__u16 available[num];

 *

 *	// Padding so a correctly-chosen num value will cache-align used_idx.

 *	char pad[sizeof(struct vring_desc) - sizeof(avail_flags)];

 *	// Padding to the next page boundary.

 *	char pad[];

 *

 *	// A ring of used descriptor heads with free-running index.

 *	__u16 used_flags;
@@ -88,18 +88,21 @@ struct vring { 
 *	struct vring_used_elem used[num];

 * };

 */

static inline void vring_init(struct vring *vr, unsigned int num, void *p)

static inline void vring_init(struct vring *vr, unsigned int num, void *p,

			      unsigned int pagesize)

{

	vr->num = num;

	vr->desc = p;

	vr->avail = p + num*sizeof(struct vring_desc);

	vr->used = p + (num+1)*(sizeof(struct vring_desc) + sizeof(__u16));

	vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + pagesize-1)

			    & ~(pagesize - 1));

}



static inline unsigned vring_size(unsigned int num)

static inline unsigned vring_size(unsigned int num, unsigned int pagesize)

{

	return (num + 1) * (sizeof(struct vring_desc) + sizeof(__u16))

		+ sizeof(__u32) + num * sizeof(struct vring_used_elem);

	return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num)

		 + pagesize - 1) & ~(pagesize - 1))

		+ sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num;

}



#ifdef __KERNEL__

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