Re: [PATCH 2/3] vhost: add support for legacy virtio

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Feb 20, 2015 at 11:14:47AM +0100, Greg Kurz wrote:
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
>  drivers/vhost/vhost.h |   20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> Michael,
> 
> The vhost_is_little_endian() helper adds unconditionnal overhead to fixed
> endian architectures: that is all architectures except arm and ppc64. This
> was addressed in QEMU with a TARGET_IS_BIENDIAN macro. Please give an
> advice about how to address this in the vhost code.
> 
> Thanks.

I suggest creating a config option for this, default to off.
When disabled the flag won't be set so userspace
can discover it's availability.


> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index ce2c68e..21e7d6a 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -176,34 +176,42 @@ static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
>  	return vq->acked_features & (1ULL << bit);
>  }
>  
> +static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
> +{
> +	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> +		return true;
> +	else
> +		return !vq->legacy_big_endian;
> +}
> +
>  /* Memory accessors */
>  static inline u16 vhost16_to_cpu(struct vhost_virtqueue *vq, __virtio16 val)
>  {
> -	return __virtio16_to_cpu(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
> +	return __virtio16_to_cpu(vhost_is_little_endian(vq), val);
>  }
>  
>  static inline __virtio16 cpu_to_vhost16(struct vhost_virtqueue *vq, u16 val)
>  {
> -	return __cpu_to_virtio16(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
> +	return __cpu_to_virtio16(vhost_is_little_endian(vq), val);
>  }
>  
>  static inline u32 vhost32_to_cpu(struct vhost_virtqueue *vq, __virtio32 val)
>  {
> -	return __virtio32_to_cpu(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
> +	return __virtio32_to_cpu(vhost_is_little_endian(vq), val);
>  }
>  
>  static inline __virtio32 cpu_to_vhost32(struct vhost_virtqueue *vq, u32 val)
>  {
> -	return __cpu_to_virtio32(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
> +	return __cpu_to_virtio32(vhost_is_little_endian(vq), val);
>  }
>  
>  static inline u64 vhost64_to_cpu(struct vhost_virtqueue *vq, __virtio64 val)
>  {
> -	return __virtio64_to_cpu(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
> +	return __virtio64_to_cpu(vhost_is_little_endian(vq), val);
>  }
>  
>  static inline __virtio64 cpu_to_vhost64(struct vhost_virtqueue *vq, u64 val)
>  {
> -	return __cpu_to_virtio64(vhost_has_feature(vq, VIRTIO_F_VERSION_1), val);
> +	return __cpu_to_virtio64(vhost_is_little_endian(vq), val);
>  }
>  #endif