Re: [PATCH] [v2] linux: virtio: Standardize virtio's concept of "page size"

[Hollis Blanchard <hollisb@us.ibm.com>]

On Thu, 2008-11-13 at 08:44 +1030, Rusty Russell wrote:
> 
> Note that I still don't have a balloon patch: want to send me one?

linux: virtio-balloon: avoid implicit use of Linux page size in balloon interface

Make the balloon interface always use 4K pages, and convert Linux pfns if
necessary. This patch assumes that Linux's PAGE_SHIFT will never be less than
12.

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
---
Only build-tested.

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -56,6 +56,14 @@ static struct virtio_device_id id_table[
 	{ 0 },
 };
 
+static u32 page_to_balloon_pfn(struct page *page)
+{
+    unsigned long pfn = page_to_pfn(page);
+
+    /* Convert pfn from Linux page size to balloon page size. */
+    return pfn >> (PAGE_SHIFT - VIRTIO_BALLOON_PFN_SHIFT);
+}
+
 static void balloon_ack(struct virtqueue *vq)
 {
 	struct virtio_balloon *vb;
@@ -99,7 +107,7 @@ static void fill_balloon(struct virtio_b
 			msleep(200);
 			break;
 		}
-		vb->pfns[vb->num_pfns] = page_to_pfn(page);
+		vb->pfns[vb->num_pfns] = page_to_balloon_pfn(page);
 		totalram_pages--;
 		vb->num_pages++;
 		list_add(&page->lru, &vb->pages);
@@ -132,7 +140,7 @@ static void leak_balloon(struct virtio_b
 	for (vb->num_pfns = 0; vb->num_pfns < num; vb->num_pfns++) {
 		page = list_first_entry(&vb->pages, struct page, lru);
 		list_del(&page->lru);
-		vb->pfns[vb->num_pfns] = page_to_pfn(page);
+		vb->pfns[vb->num_pfns] = page_to_balloon_pfn(page);
 		vb->num_pages--;
 	}
 
diff --git a/include/linux/virtio_balloon.h b/include/linux/virtio_balloon.h
--- a/include/linux/virtio_balloon.h
+++ b/include/linux/virtio_balloon.h
@@ -10,6 +10,9 @@
 /* The feature bitmap for virtio balloon */
 #define VIRTIO_BALLOON_F_MUST_TELL_HOST	0 /* Tell before reclaiming pages */
 
+/* Size of a PFN in the balloon interface. */
+#define VIRTIO_BALLOON_PFN_SHIFT 12
+
 struct virtio_balloon_config
 {
 	/* Number of pages host wants Guest to give up. */


-- 
Hollis Blanchard
IBM Linux Technology Center