Re: [PATCH v5 6/8] 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 <redacted>
---

Changes since v4:
- style fixes (I have chosen if ... else in most places to stay below
  80 columns, with the notable exception of the vhost helper which gets
  shorten in a later patch)

 drivers/net/macvtap.c            |    5 ++++-
 drivers/net/tun.c                |    5 ++++-
 drivers/vhost/vhost.h            |    2 +-
 include/linux/virtio_byteorder.h |   24 ++++++++++++++----------
 include/linux/virtio_config.h    |    5 ++++-
 include/linux/vringh.h           |    2 +-
 6 files changed, 28 insertions(+), 15 deletions(-)

diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
index a2f2958..6cf6b3e 100644
--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c

@@ -51,7 +51,10 @@ struct macvtap_queue {
 
 static inline bool macvtap_is_little_endian(struct macvtap_queue *q)
 {
-	return q->flags & MACVTAP_VNET_LE;
+	if (q->flags & MACVTAP_VNET_LE)
+		return true;
+	else
+		return virtio_legacy_is_little_endian();

 }
 
 static inline u16 macvtap16_to_cpu(struct macvtap_queue *q, __virtio16 val)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 3c3d6c0..5b044d4 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c

@@ -208,7 +208,10 @@ struct tun_struct {
 
 static inline bool tun_is_little_endian(struct tun_struct *tun)
 {
-	return tun->flags & TUN_VNET_LE;
+	if (tun->flags & TUN_VNET_LE)
+		return true;
+	else
+		return virtio_legacy_is_little_endian();

 }
 
 static inline u16 tun16_to_cpu(struct tun_struct *tun, __virtio16 val)

diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 6a49960..954c657 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h

@@ -175,7 +175,7 @@ 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) ? true : virtio_legacy_is_little_endian();
 }