Re: [Qemu-devel] [PATCH v4 5/6] vhost: simplify vhost_needs_vring_endian()

[Laurent Vivier <lvivier@redhat.com>]

On 05/02/2016 11:46, Greg Kurz wrote:
> After the call to virtio_vdev_has_feature(), we only care for legacy
> devices, so we don't need the extra check in virtio_is_big_endian().
> 
> Also the device_endian field is always set (VIRTIO_DEVICE_ENDIAN_UNKNOWN
> may only happen on a virtio_load() path that cannot lead here), so we
> don't need the assert() either.
> 
> This open codes the device_endian checking in vhost_needs_vring_endian().
> It also adds a comment to explain the logic, as recent reviews showed the
> cross-endian tweaks aren't that obvious.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>

Reviewed-by: Laurent Vivier <lvivier@redhat.com>

> ---
>  hw/virtio/vhost.c |    9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index bb17177f5e7e..9f8ac38ccf2d 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -749,6 +749,11 @@ static void vhost_log_stop(MemoryListener *listener,
>      /* FIXME: implement */
>  }
>  
> +/* The vhost driver natively knows how to handle the vrings of non
> + * cross-endian legacy devices and modern devices. Only legacy devices
> + * exposed to a bi-endian guest may require the vhost driver to use a
> + * specific endianness.
> + */
>  static inline bool vhost_needs_vring_endian(VirtIODevice *vdev)
>  {
>      if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
> @@ -756,9 +761,9 @@ static inline bool vhost_needs_vring_endian(VirtIODevice *vdev)
>      }
>  #ifdef TARGET_IS_BIENDIAN
>  #ifdef HOST_WORDS_BIGENDIAN
> -    return !virtio_is_big_endian(vdev);
> +    return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_LITTLE;
>  #else
> -    return virtio_is_big_endian(vdev);
> +    return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_BIG;
>  #endif
>  #else
>      return false;
> 
>