Re: [PATCH 1/3] vhost: add VHOST_VRING_F_LEGACY_BIG_ENDIAN flag

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

On Fri, Feb 20, 2015 at 11:07:39AM +0100, Greg Kurz wrote:
> The VHOST_VRING_F_LEGACY_BIG_ENDIAN flag informs the kernel that the
> associated device is big endian. Of course, this only makes sense for
> legacy virtio devices since modern virtio devices are always little
> endian.
> 
> It will be used by the vhost memory accessors to byteswap vring data when
> we have a legacy device, in case host and guest endianness differ.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
>  drivers/vhost/vhost.c      |    6 +++++-
>  drivers/vhost/vhost.h      |    3 +++
>  include/uapi/linux/vhost.h |    2 ++
>  3 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 2ee2826..dad3c37 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -199,6 +199,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  	vq->call = NULL;
>  	vq->log_ctx = NULL;
>  	vq->memory = NULL;
> +	vq->legacy_big_endian = false;
>  }
>  
>  static int vhost_worker(void *data)
> @@ -701,7 +702,8 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
>  			r = -EFAULT;
>  			break;
>  		}
> -		if (a.flags & ~(0x1 << VHOST_VRING_F_LOG)) {
> +		if (a.flags & ~(0x1 << VHOST_VRING_F_LOG|
> +				0x1 << VHOST_VRING_F_LEGACY_BIG_ENDIAN)) {

whitespace damage here

>  			r = -EOPNOTSUPP;
>  			break;
>  		}

need to also make sure LEGACY_BIG_ENDIAN isn't set with VERSION_1.


> @@ -751,6 +753,8 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
>  		vq->avail = (void __user *)(unsigned long)a.avail_user_addr;
>  		vq->log_addr = a.log_guest_addr;
>  		vq->used = (void __user *)(unsigned long)a.used_user_addr;
> +		vq->legacy_big_endian =
> +			!!(a.flags & (0x1 << VHOST_VRING_F_LEGACY_BIG_ENDIAN));
>  		break;
>  	case VHOST_SET_VRING_KICK:
>  		if (copy_from_user(&f, argp, sizeof f)) {
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 8c1c792..ce2c68e 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -106,6 +106,9 @@ struct vhost_virtqueue {
>  	/* Log write descriptors */
>  	void __user *log_base;
>  	struct vhost_log *log;
> +
> +	/* We need to know the device endianness with legacy virtio. */
> +	bool legacy_big_endian;
>  };
>  
>  struct vhost_dev {
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index bb6a5b4..0bf4491 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -34,6 +34,8 @@ struct vhost_vring_addr {
>  	/* Flag values: */
>  	/* Whether log address is valid. If set enables logging. */
>  #define VHOST_VRING_F_LOG 0
> +	/* Whether we have a big-endian legacy virtio device. */
> +#define VHOST_VRING_F_LEGACY_BIG_ENDIAN 1
>  
>  	/* Start of array of descriptors (virtually contiguous) */
>  	__u64 desc_user_addr;