[Qemu-devel] [PATCH 6/6] virtio: optimize virtio_access_is_big_endian() for little-endian targets

[Greg Kurz]

When adding cross-endian support, we introduced the TARGET_IS_BIENDIAN macro
and the virtio_access_is_big_endian() helper to have a branchless fast path
in the virtio memory accessors for targets that don't switch endian.

This was considered as a strong requirement at the time.

Now we have added a runtime check for virtio 1.0, which ruins the benefit
of the virtio_access_is_big_endian() helper for always little-endian targets.

With this patch, always little-endian targets stop checking for virtio 1.0,
since the result is little-endian in all cases.

Signed-off-by: Greg Kurz <address@hidden>
---
 include/hw/virtio/virtio-access.h |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/hw/virtio/virtio-access.h 
b/include/hw/virtio/virtio-access.h
index f1f12afe9089..fba4b4a4e1b2 100644
--- a/include/hw/virtio/virtio-access.h
+++ b/include/hw/virtio/virtio-access.h
@@ -19,10 +19,13 @@
 
 static inline bool virtio_access_is_big_endian(VirtIODevice *vdev)
 {
+#if defined(TARGET_IS_BIENDIAN) || defined(TARGET_WORDS_BIGENDIAN)
     if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
         /* Devices conforming to VIRTIO 1.0 or later are always LE. */
         return false;
     }
+#endif
+
 #if defined(TARGET_IS_BIENDIAN)
     return virtio_is_big_endian(vdev);
 #elif defined(TARGET_WORDS_BIGENDIAN)