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virtio: optimize virtio_access_is_big_endian() for little-endian targets 

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.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
This commit is contained in:
Greg Kurz 2016-02-05 11:46:18 +01:00 committed by Michael S. Tsirkin
parent 46f70ff148
commit e5157e313c
1 changed files with 3 additions and 3 deletions
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6
include/hw/virtio/virtio-access.h
View File

@ -19,13 +19,13 @@
static inline bool virtio_access_is_big_endian(VirtIODevice *vdev)
{
#if defined(TARGET_IS_BIENDIAN)
return virtio_is_big_endian(vdev);
#elif 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;
}
#if defined(TARGET_IS_BIENDIAN)
return virtio_is_big_endian(vdev);
#elif defined(TARGET_WORDS_BIGENDIAN)
return true;
#else
return false;