virtio: add explicit big-endian support to memory accessors

The current memory accessors logic is:
- little endian if little_endian
- native endian (i.e. no byteswap) if !little_endian

If we want to fully support cross-endian vhost, we also need to be
able to convert to big endian.

Instead of changing the little_endian argument to some 3-value enum, this
patch changes the logic to:
- little endian if little_endian
- big endian if !little_endian

The native endian case is handled by all users with a trivial helper. This
patch doesn't change any functionality, nor it does add overhead.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

drivers/net/macvtap.c

@@ -51,7 +51,8 @@ struct macvtap_queue {
 
 static inline bool macvtap_is_little_endian(struct macvtap_queue *q)
 {
-	return q->flags & MACVTAP_VNET_LE;
+	return q->flags & MACVTAP_VNET_LE ||
+		virtio_legacy_is_little_endian();
 }
 
 static inline u16 macvtap16_to_cpu(struct macvtap_queue *q, __virtio16 val)

drivers/net/tun.c

@@ -208,7 +208,8 @@ struct tun_struct {
 
 static inline bool tun_is_little_endian(struct tun_struct *tun)
 {
-	return tun->flags & TUN_VNET_LE;
+	return tun->flags & TUN_VNET_LE ||
+		virtio_legacy_is_little_endian();
 }
 
 static inline u16 tun16_to_cpu(struct tun_struct *tun, __virtio16 val)

drivers/vhost/vhost.h

@@ -175,7 +175,8 @@ static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
 
 static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
 {
-	return vhost_has_feature(vq, VIRTIO_F_VERSION_1);
+	return vhost_has_feature(vq, VIRTIO_F_VERSION_1) ||
+		virtio_legacy_is_little_endian();
 }
 
 /* Memory accessors */

include/linux/virtio_byteorder.h

@@ -3,57 +3,61 @@
 #include <linux/types.h>
 #include <uapi/linux/virtio_types.h>
 
-/*
- * Low-level memory accessors for handling virtio in modern little endian and in
- * compatibility native endian format.
- */
+static inline bool virtio_legacy_is_little_endian(void)
+{
+#ifdef __LITTLE_ENDIAN
+	return true;
+#else
+	return false;
+#endif
+}
 
 static inline u16 __virtio16_to_cpu(bool little_endian, __virtio16 val)
 {
 	if (little_endian)
 		return le16_to_cpu((__force __le16)val);
 	else
-		return (__force u16)val;
+		return be16_to_cpu((__force __be16)val);
 }
 
 static inline __virtio16 __cpu_to_virtio16(bool little_endian, u16 val)
 {
 	if (little_endian)
 		return (__force __virtio16)cpu_to_le16(val);
 	else
-		return (__force __virtio16)val;
+		return (__force __virtio16)cpu_to_be16(val);
 }
 
 static inline u32 __virtio32_to_cpu(bool little_endian, __virtio32 val)
 {
 	if (little_endian)
 		return le32_to_cpu((__force __le32)val);
 	else
-		return (__force u32)val;
+		return be32_to_cpu((__force __be32)val);
 }
 
 static inline __virtio32 __cpu_to_virtio32(bool little_endian, u32 val)
 {
 	if (little_endian)
 		return (__force __virtio32)cpu_to_le32(val);
 	else
-		return (__force __virtio32)val;
+		return (__force __virtio32)cpu_to_be32(val);
 }
 
 static inline u64 __virtio64_to_cpu(bool little_endian, __virtio64 val)
 {
 	if (little_endian)
 		return le64_to_cpu((__force __le64)val);
 	else
-		return (__force u64)val;
+		return be64_to_cpu((__force __be64)val);
 }
 
 static inline __virtio64 __cpu_to_virtio64(bool little_endian, u64 val)
 {
 	if (little_endian)
 		return (__force __virtio64)cpu_to_le64(val);
 	else
-		return (__force __virtio64)val;
+		return (__force __virtio64)cpu_to_be64(val);
 }
 
 #endif /* _LINUX_VIRTIO_BYTEORDER */

include/linux/virtio_config.h

@@ -207,7 +207,8 @@ int virtqueue_set_affinity(struct virtqueue *vq, int cpu)
 
 static inline bool virtio_is_little_endian(struct virtio_device *vdev)
 {
-	return virtio_has_feature(vdev, VIRTIO_F_VERSION_1);
+	return virtio_has_feature(vdev, VIRTIO_F_VERSION_1) ||
+		virtio_legacy_is_little_endian();
 }
 
 /* Memory accessors */

include/linux/vringh.h

@@ -228,7 +228,8 @@ static inline void vringh_notify(struct vringh *vrh)
 
 static inline bool vringh_is_little_endian(const struct vringh *vrh)
 {
-	return vrh->little_endian;
+	return vrh->little_endian ||
+		virtio_legacy_is_little_endian();
 }
 
 static inline u16 vringh16_to_cpu(const struct vringh *vrh, __virtio16 val)