Re: [PATCH 1/2] vhost: helpers to enable/disable vring endianness

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

On Wed, Jan 13, 2016 at 06:09:41PM +0100, Greg Kurz wrote:
> The default use case for vhost is when the host and the vring have the
> same endianness (default native endianness). But there are cases where
> they differ and vhost should byteswap when accessing the vring:
> - the host is big endian and the vring comes from a virtio 1.0 device
>   which is always little endian
> - the architecture is bi-endian and the vring comes from a legacy virtio
>   device with a different endianness than the endianness of the host (aka
>   legacy cross-endian)
> 
> These cases are handled by the vq->is_le and the optional vq->user_be,
> with the following logic:
> - if none of the fields is enabled, vhost access the vring without byteswap
> - if the vring is virtio 1.0 and the host is big endian, vq->is_le is
>   enabled to enforce little endian access to the vring
> - if the vring is legacy cross-endian, userspace enables vq->user_be
>   to inform vhost about the vring endianness. This endianness is then
>   enforced for vring accesses through vq->is_le again
> 
> The logic is unclear in the current code.
> 
> This patch introduces helpers with explicit enable and disable semantics,
> for better clarity.
> 
> No behaviour change.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
>  drivers/vhost/vhost.c |   28 +++++++++++++++++++---------
>  1 file changed, 19 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index ad2146a9ab2d..e02e06755ab7 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -43,11 +43,16 @@ enum {
>  #define vhost_avail_event(vq) ((__virtio16 __user *)&vq->used->ring[vq->num])
>  
>  #ifdef CONFIG_VHOST_CROSS_ENDIAN_LEGACY
> -static void vhost_vq_reset_user_be(struct vhost_virtqueue *vq)
> +static void vhost_disable_user_be(struct vhost_virtqueue *vq)
>  {
>  	vq->user_be = !virtio_legacy_is_little_endian();
>  }
>  

Hmm this doesn't look like an improvement to me.
What does it mean to disable big endian? Make it little endian?
Existing reset seems to make sense.

> +static void vhost_enable_user_be(struct vhost_virtqueue *vq, bool user_be)
> +{
> +	vq->user_be = user_be;
> +}
> +

And this is maybe "init_user_be"?

>  static long vhost_set_vring_endian(struct vhost_virtqueue *vq, int __user *argp)
>  {
>  	struct vhost_vring_state s;
> @@ -62,7 +67,7 @@ static long vhost_set_vring_endian(struct vhost_virtqueue *vq, int __user *argp)
>  	    s.num != VHOST_VRING_BIG_ENDIAN)
>  		return -EINVAL;
>  
> -	vq->user_be = s.num;
> +	vhost_enable_user_be(vq, !!s.num);
>  
>  	return 0;
>  }
> @@ -81,7 +86,7 @@ static long vhost_get_vring_endian(struct vhost_virtqueue *vq, u32 idx,
>  	return 0;
>  }
>  
> -static void vhost_init_is_le(struct vhost_virtqueue *vq)
> +static void vhost_enable_is_le(struct vhost_virtqueue *vq)
>  {
>  	/* Note for legacy virtio: user_be is initialized at reset time
>  	 * according to the host endianness. If userspace does not set an

Same thing really. I'd rather add "reset_is_le".

> @@ -91,7 +96,7 @@ static void vhost_init_is_le(struct vhost_virtqueue *vq)
>  	vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) || !vq->user_be;
>  }
>  #else
> -static void vhost_vq_reset_user_be(struct vhost_virtqueue *vq)
> +static void vhost_disable_user_be(struct vhost_virtqueue *vq)
>  {
>  }
>  
> @@ -106,13 +111,18 @@ static long vhost_get_vring_endian(struct vhost_virtqueue *vq, u32 idx,
>  	return -ENOIOCTLCMD;
>  }
>  
> -static void vhost_init_is_le(struct vhost_virtqueue *vq)
> +static void vhost_enable_is_le(struct vhost_virtqueue *vq)
>  {
>  	if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
>  		vq->is_le = true;
>  }
>  #endif /* CONFIG_VHOST_CROSS_ENDIAN_LEGACY */
>  
> +static void vhost_disable_is_le(struct vhost_virtqueue *vq)
> +{
> +	vq->is_le = virtio_legacy_is_little_endian();
> +}
> +
>  static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
>  			    poll_table *pt)
>  {
> @@ -276,8 +286,8 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  	vq->call = NULL;
>  	vq->log_ctx = NULL;
>  	vq->memory = NULL;
> -	vq->is_le = virtio_legacy_is_little_endian();
> -	vhost_vq_reset_user_be(vq);
> +	vhost_disable_is_le(vq);
> +	vhost_disable_user_be(vq);
>  }
>  
>  static int vhost_worker(void *data)
> @@ -1157,11 +1167,11 @@ int vhost_init_used(struct vhost_virtqueue *vq)
>  	__virtio16 last_used_idx;
>  	int r;
>  	if (!vq->private_data) {
> -		vq->is_le = virtio_legacy_is_little_endian();
> +		vhost_disable_is_le(vq);
>  		return 0;
>  	}
>  
> -	vhost_init_is_le(vq);
> +	vhost_enable_is_le(vq);
>  
>  	r = vhost_update_used_flags(vq);
>  	if (r)