virtio: use LGUEST_VRING_ALIGN instead of relying on pagesize

This doesn't really matter, since lguest is i386 only at the moment,
but we could actually choose a different value. (lguest doesn't have
a guarenteed ABI).

Signed-off-by: Rusty Russell <[e-mail]>

Documentation/lguest/lguest.c

···
 		/* Zero out the virtqueues. */
 		for (vq = dev->vq; vq; vq = vq->next) {
 			memset(vq->vring.desc, 0,
-			       vring_size(vq->config.num, getpagesize()));
+			       vring_size(vq->config.num, LGUEST_VRING_ALIGN));
 			lg_last_avail(vq) = 0;
 		}
 	} else if (dev->desc->status & VIRTIO_CONFIG_S_FAILED) {
···
 	void *p;
 
 	/* First we need some memory for this virtqueue. */
-	pages = (vring_size(num_descs, getpagesize()) + getpagesize() - 1)
+	pages = (vring_size(num_descs, LGUEST_VRING_ALIGN) + getpagesize() - 1)
 		/ getpagesize();
 	p = get_pages(pages);
 
···
 	vq->config.pfn = to_guest_phys(p) / getpagesize();
 
 	/* Initialize the vring. */
-	vring_init(&vq->vring, num_descs, p, getpagesize());
+	vring_init(&vq->vring, num_descs, p, LGUEST_VRING_ALIGN);
 
 	/* Append virtqueue to this device's descriptor.  We use
 	 * device_config() to get the end of the device's current virtqueues;

drivers/lguest/lguest_device.c

···
 	/* 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,
-							PAGE_SIZE),
+							LGUEST_VRING_ALIGN),
 					     PAGE_SIZE));
 	if (!lvq->pages) {
 		err = -ENOMEM;

include/linux/lguest_launcher.h

···
 	LHREQ_IRQ, /* + irq */
 	LHREQ_BREAK, /* + on/off flag (on blocks until someone does off) */
 };
+
+/* The alignment to use between consumer and producer parts of vring.
+ * x86 pagesize for historical reasons. */
+#define LGUEST_VRING_ALIGN	4096
 #endif /* _LINUX_LGUEST_LAUNCHER */