virtio_balloon: transitional interface

Virtio 1.0 doesn't include a modern balloon device.
But it's not a big change to support a transitional
balloon device: this has the advantage of supporting
existing drivers, transparently.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

2 files changed, 43 insertions, 10 deletions

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 6a356e344f82..82e80e034f25 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -214,8 +214,8 @@ static inline void update_stat(struct virtio_balloon *vb, int idx,
                                u16 tag, u64 val)
 {
         BUG_ON(idx >= VIRTIO_BALLOON_S_NR);
-        vb->stats[idx].tag = tag;
-        vb->stats[idx].val = val;
+        vb->stats[idx].tag = cpu_to_virtio16(vb->vdev, tag);
+        vb->stats[idx].val = cpu_to_virtio64(vb->vdev, val);
 }
 
 #define pages_to_bytes(x) ((u64)(x) << PAGE_SHIFT)
@@ -283,18 +283,27 @@ static void virtballoon_changed(struct virtio_device *vdev)
 
 static inline s64 towards_target(struct virtio_balloon *vb)
 {
-        __le32 v;
         s64 target;
+        u32 num_pages;
 
-        virtio_cread(vb->vdev, struct virtio_balloon_config, num_pages, &v);
+        virtio_cread(vb->vdev, struct virtio_balloon_config, num_pages,
+                     &num_pages);
 
-        target = le32_to_cpu(v);
+        /* Legacy balloon config space is LE, unlike all other devices. */
+        if (!virtio_has_feature(vb->vdev, VIRTIO_F_VERSION_1))
+                num_pages = le32_to_cpu((__force __le32)num_pages);
+
+        target = num_pages;
         return target - vb->num_pages;
 }
 
 static void update_balloon_size(struct virtio_balloon *vb)
 {
-        __le32 actual = cpu_to_le32(vb->num_pages);
+        u32 actual = vb->num_pages;
+
+        /* Legacy balloon config space is LE, unlike all other devices. */
+        if (!virtio_has_feature(vb->vdev, VIRTIO_F_VERSION_1))
+                actual = (__force u32)cpu_to_le32(actual);
 
         virtio_cwrite(vb->vdev, struct virtio_balloon_config, actual,
                       &actual);
diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
index 4b0488f20b2e..984169a819ee 100644
--- a/include/uapi/linux/virtio_balloon.h
+++ b/include/uapi/linux/virtio_balloon.h
@@ -25,6 +25,7 @@
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE. */
+#include <linux/types.h>
 #include <linux/virtio_ids.h>
 #include <linux/virtio_config.h>
 
@@ -38,9 +39,9 @@
 
 struct virtio_balloon_config {
         /* Number of pages host wants Guest to give up. */
-        __le32 num_pages;
+        __u32 num_pages;
         /* Number of pages we've actually got in balloon. */
-        __le32 actual;
+        __u32 actual;
 };
 
 #define VIRTIO_BALLOON_S_SWAP_IN  0   /* Amount of memory swapped in */
@@ -51,9 +52,32 @@ struct virtio_balloon_config {
 #define VIRTIO_BALLOON_S_MEMTOT   5   /* Total amount of memory */
 #define VIRTIO_BALLOON_S_NR       6
 
+/*
+ * Memory statistics structure.
+ * Driver fills an array of these structures and passes to device.
+ *
+ * NOTE: fields are laid out in a way that would make compiler add padding
+ * between and after fields, so we have to use compiler-specific attributes to
+ * pack it, to disable this padding. This also often causes compiler to
+ * generate suboptimal code.
+ *
+ * We maintain this statistics structure format for backwards compatibility,
+ * but don't follow this example.
+ *
+ * If implementing a similar structure, do something like the below instead:
+ *     struct virtio_balloon_stat {
+ *         __virtio16 tag;
+ *         __u8 reserved[6];
+ *         __virtio64 val;
+ *     };
+ *
+ * In other words, add explicit reserved fields to align field and
+ * structure boundaries at field size, avoiding compiler padding
+ * without the packed attribute.
+ */
 struct virtio_balloon_stat {
-        __u16 tag;
-        __u64 val;
+        __virtio16 tag;
+        __virtio64 val;
 } __attribute__((packed));
 
 #endif /* _LINUX_VIRTIO_BALLOON_H */