Re: [PATCH v4 6/8] virtio: add explicit big-endian support to memory accessors

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

On Fri, Apr 10, 2015 at 12:16:20PM +0200, Greg Kurz wrote:
> The current memory accessors logic is:
> - little endian if little_endian
> - native endian (i.e. no byteswap) if !little_endian
> 
> If we want to fully support cross-endian vhost, we also need to be
> able to convert to big endian.
> 
> Instead of changing the little_endian argument to some 3-value enum, this
> patch changes the logic to:
> - little endian if little_endian
> - big endian if !little_endian
> 
> The native endian case is handled by all users with a trivial helper. This
> patch doesn't change any functionality, nor it does add overhead.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---

OK overall. Style comment:

>  drivers/net/macvtap.c            |    4 +++-
>  drivers/net/tun.c                |    4 +++-
>  drivers/vhost/vhost.h            |    4 +++-
>  include/linux/virtio_byteorder.h |   24 ++++++++++++++----------
>  include/linux/virtio_config.h    |    4 +++-
>  include/linux/vringh.h           |    4 +++-
>  6 files changed, 29 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> index a2f2958..0a03a66 100644
> --- a/drivers/net/macvtap.c
> +++ b/drivers/net/macvtap.c
> @@ -51,7 +51,9 @@ struct macvtap_queue {
>  
>  static inline bool macvtap_is_little_endian(struct macvtap_queue *q)
>  {
> -	return q->flags & MACVTAP_VNET_LE;
> +	if (q->flags & MACVTAP_VNET_LE)
> +		return true;
> +	return virtio_legacy_is_little_endian();
>  }
>  

I'd prefer a bit more symmetry:

+	if (q->flags & MACVTAP_VNET_LE)
+		return true;
+	else
+		return virtio_legacy_is_little_endian();

Or better just:
	return (q->flags & MACVTAP_VNET_LE) ? true : virtio_legacy_is_little_endian();

might make line long, but your follow-up patch makes it short again,
so that's ok.


>  static inline u16 macvtap16_to_cpu(struct macvtap_queue *q, __virtio16 val)
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 3c3d6c0..053f9b6 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -208,7 +208,9 @@ struct tun_struct {
>  
>  static inline bool tun_is_little_endian(struct tun_struct *tun)
>  {
> -	return tun->flags & TUN_VNET_LE;
> +	if (tun->flags & TUN_VNET_LE)
> +		return true;
> +	return virtio_legacy_is_little_endian();
>  }
>  
>  static inline u16 tun16_to_cpu(struct tun_struct *tun, __virtio16 val)
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 6a49960..4e9a186 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -175,7 +175,9 @@ static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
>  
>  static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
>  {
> -	return vhost_has_feature(vq, VIRTIO_F_VERSION_1);
> +	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> +		return true;
> +	return virtio_legacy_is_little_endian();
>  }
>  
>  /* Memory accessors */
> diff --git a/include/linux/virtio_byteorder.h b/include/linux/virtio_byteorder.h
> index 51865d0..ce63a2c 100644
> --- a/include/linux/virtio_byteorder.h
> +++ b/include/linux/virtio_byteorder.h
> @@ -3,17 +3,21 @@
>  #include <linux/types.h>
>  #include <uapi/linux/virtio_types.h>
>  
> -/*
> - * Low-level memory accessors for handling virtio in modern little endian and in
> - * compatibility native endian format.
> - */
> +static inline bool virtio_legacy_is_little_endian(void)
> +{
> +#ifdef __LITTLE_ENDIAN
> +	return true;
> +#else
> +	return false;
> +#endif
> +}
>  
>  static inline u16 __virtio16_to_cpu(bool little_endian, __virtio16 val)
>  {
>  	if (little_endian)
>  		return le16_to_cpu((__force __le16)val);
>  	else
> -		return (__force u16)val;
> +		return be16_to_cpu((__force __be16)val);
>  }
>  
>  static inline __virtio16 __cpu_to_virtio16(bool little_endian, u16 val)
> @@ -21,7 +25,7 @@ static inline __virtio16 __cpu_to_virtio16(bool little_endian, u16 val)
>  	if (little_endian)
>  		return (__force __virtio16)cpu_to_le16(val);
>  	else
> -		return (__force __virtio16)val;
> +		return (__force __virtio16)cpu_to_be16(val);
>  }
>  
>  static inline u32 __virtio32_to_cpu(bool little_endian, __virtio32 val)
> @@ -29,7 +33,7 @@ static inline u32 __virtio32_to_cpu(bool little_endian, __virtio32 val)
>  	if (little_endian)
>  		return le32_to_cpu((__force __le32)val);
>  	else
> -		return (__force u32)val;
> +		return be32_to_cpu((__force __be32)val);
>  }
>  
>  static inline __virtio32 __cpu_to_virtio32(bool little_endian, u32 val)
> @@ -37,7 +41,7 @@ static inline __virtio32 __cpu_to_virtio32(bool little_endian, u32 val)
>  	if (little_endian)
>  		return (__force __virtio32)cpu_to_le32(val);
>  	else
> -		return (__force __virtio32)val;
> +		return (__force __virtio32)cpu_to_be32(val);
>  }
>  
>  static inline u64 __virtio64_to_cpu(bool little_endian, __virtio64 val)
> @@ -45,7 +49,7 @@ static inline u64 __virtio64_to_cpu(bool little_endian, __virtio64 val)
>  	if (little_endian)
>  		return le64_to_cpu((__force __le64)val);
>  	else
> -		return (__force u64)val;
> +		return be64_to_cpu((__force __be64)val);
>  }
>  
>  static inline __virtio64 __cpu_to_virtio64(bool little_endian, u64 val)
> @@ -53,7 +57,7 @@ static inline __virtio64 __cpu_to_virtio64(bool little_endian, u64 val)
>  	if (little_endian)
>  		return (__force __virtio64)cpu_to_le64(val);
>  	else
> -		return (__force __virtio64)val;
> +		return (__force __virtio64)cpu_to_be64(val);
>  }
>  
>  #endif /* _LINUX_VIRTIO_BYTEORDER */
> diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> index bd1a582..36a6daa 100644
> --- a/include/linux/virtio_config.h
> +++ b/include/linux/virtio_config.h
> @@ -207,7 +207,9 @@ int virtqueue_set_affinity(struct virtqueue *vq, int cpu)
>  
>  static inline bool virtio_is_little_endian(struct virtio_device *vdev)
>  {
> -	return virtio_has_feature(vdev, VIRTIO_F_VERSION_1);
> +	if (virtio_has_feature(vdev, VIRTIO_F_VERSION_1))
> +		return true;
> +	return virtio_legacy_is_little_endian();
>  }
>  
>  /* Memory accessors */
> diff --git a/include/linux/vringh.h b/include/linux/vringh.h
> index 3ed62ef..d786c2d 100644
> --- a/include/linux/vringh.h
> +++ b/include/linux/vringh.h
> @@ -228,7 +228,9 @@ static inline void vringh_notify(struct vringh *vrh)
>  
>  static inline bool vringh_is_little_endian(const struct vringh *vrh)
>  {
> -	return vrh->little_endian;
> +	if (vrh->little_endian)
> +		return true;
> +	return virtio_legacy_is_little_endian();
>  }
>  
>  static inline u16 vringh16_to_cpu(const struct vringh *vrh, __virtio16 val)