Re: [Qemu-devel] [PATCH 3/3] virtio: optimize virtio_access_is_big_endian() for little-endian targets

[Cornelia Huck]

On Mon, 09 Nov 2015 18:58:34 +0100
Greg Kurz <address@hidden> 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, fixed little-endian targets stop checking for virtio 1.0,
> since the result is little-endian in all cases. 

So always-LE gets optimized, while always-BE and bi-endian stay the same?

(Is there a measurable impact?)

> The helper also gets renamed
> so it is clear it is optimized for fast paths.

Even if it isn't actually 'fast' on anything other than fixed-LE?
> 
> Signed-off-by: Greg Kurz <address@hidden>
> ---
>  include/hw/virtio/virtio-access.h |   43 
> ++++++++++++++++++++-----------------
>  1 file changed, 23 insertions(+), 20 deletions(-)