virtio: 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>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (modified)

2 files changed, 14 insertions, 2 deletions

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 62eab43152d..59268266b79 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -56,6 +56,15 @@ static struct virtio_device_id id_table[] = {
         { 0 },
 };
 
+static u32 page_to_balloon_pfn(struct page *page)
+{
+        unsigned long pfn = page_to_pfn(page);
+
+        BUILD_BUG_ON(PAGE_SHIFT < VIRTIO_BALLOON_PFN_SHIFT);
+        /* 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 +108,7 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
                         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 +141,7 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
         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
index c30c7bfbf39..8726ff77763 100644
--- 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. */