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

[Laurent Vivier]

On 07/01/2016 12:32, 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 <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


virtio_is_big_endian() checks VIRTIO_F_VERSION_1, so you don't need to
check it here if TARGET_IS_BIENDIAN is defined.

> +
>  #if defined(TARGET_IS_BIENDIAN)
>      return virtio_is_big_endian(vdev);
>  #elif defined(TARGET_WORDS_BIGENDIAN)
> 
> 
So you can move virtio_vdev_has_feature() here.

Laurent