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Re: [Qemu-devel] [PATCH v2 5/5] virtio: optimize virtio_access_is_big_en
From: |
Laurent Vivier |
Subject: |
Re: [Qemu-devel] [PATCH v2 5/5] virtio: optimize virtio_access_is_big_endian() for little-endian targets |
Date: |
Tue, 12 Jan 2016 01:33:47 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0 |
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 <address@hidden>
> ---
> 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 <address@hidden>