Re: [Qemu-devel] [PATCH v2 5/5] virtio: optimize virtio_access_is_big_endian() for little-endian targets

[Laurent Vivier <lvivier@redhat.com>]

On 11/01/2016 17:18, Greg Kurz wrote:
> 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 <gkurz@linux.vnet.ibm.com>
> ---
> v2:
> - simplified the logic as suggested by Laurent Vivier
> ---
>  include/hw/virtio/virtio-access.h |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/hw/virtio/virtio-access.h b/include/hw/virtio/virtio-access.h
> index f1f12afe9089..8dc84f520316 100644
> --- a/include/hw/virtio/virtio-access.h
> +++ b/include/hw/virtio/virtio-access.h
> @@ -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;
> 
> 
Reviewed-by: Laurent Vivier <lvivier@redhat.com>