Re: [PATCH] vhost: move is_le setup to the backend

[Michael S. Tsirkin]

On Mon, Jan 11, 2016 at 03:39:38PM +0100, Greg Kurz wrote:
> The vq->is_le field is used to fix endianness when accessing the vring via
> the cpu_to_vhost16() and vhost16_to_cpu() helpers in the following cases:
> 
> 1) host is big endian and device is modern virtio
> 
> 2) host has cross-endian support and device is legacy virtio with a different
>    endianness than the host
> 
> Both cases rely on the VHOST_SET_FEATURES ioctl, but 2) also needs the
> VHOST_SET_VRING_ENDIAN ioctl to be called by userspace. Since vq->is_le
> is only needed when the backend is active, it was decided to set it at
> backend start.
> 
> This is currently done in vhost_init_used()->vhost_init_is_le() but it
> obfuscates the core vhost code. This patch moves the is_le setup to a
> dedicated function that is called from the backend code.
> 
> Note vhost_net is the only backend that can pass vq->private_data == NULL to
> vhost_init_used(), hence the "if (sock)" branch.
> 
> No behaviour change.
> 
> Reviewed-by: Cornelia Huck <cornelia.huck@xxxxxxxxxx>
> Signed-off-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx>
> ---
> 
> Hi Michael,
> 
> This is a cleanup patch I had posted last year:
> 
> http://patchwork.ozlabs.org/patch/538277/
> 
> I am just reposting with Cornelia's R-b.
> 
> Cheers.
> 
> --
> Greg
> 
>  drivers/vhost/net.c   |    6 ++++++
>  drivers/vhost/scsi.c  |    3 +++
>  drivers/vhost/test.c  |    2 ++
>  drivers/vhost/vhost.c |   12 +++++++-----
>  drivers/vhost/vhost.h |    1 +
>  5 files changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 9eda69e40678..d6319cb2664c 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -917,6 +917,12 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
>  
>  		vhost_net_disable_vq(n, vq);
>  		vq->private_data = sock;
> +
> +		if (sock)
> +			vhost_set_is_le(vq);
> +		else
> +			vq->is_le = virtio_legacy_is_little_endian();
> +
>  		r = vhost_init_used(vq);
>  		if (r)
>  			goto err_used;

This part is kind of ugly. I think it's cleaner is the generic code.
How about we teach vhost_set_is_le to test vq->private_data and DTRT?

> diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
> index 29cfc57d496e..1f4f405ebba8 100644
> --- a/drivers/vhost/scsi.c
> +++ b/drivers/vhost/scsi.c
> @@ -1274,6 +1274,9 @@ vhost_scsi_set_endpoint(struct vhost_scsi *vs,
>  			vq = &vs->vqs[i].vq;
>  			mutex_lock(&vq->mutex);
>  			vq->private_data = vs_tpg;
> +
> +			vhost_set_is_le(vq);
> +
>  			vhost_init_used(vq);
>  			mutex_unlock(&vq->mutex);
>  		}
> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> index f2882ac98726..b1c7df502211 100644
> --- a/drivers/vhost/test.c
> +++ b/drivers/vhost/test.c
> @@ -196,6 +196,8 @@ static long vhost_test_run(struct vhost_test *n, int test)
>  		oldpriv = vq->private_data;
>  		vq->private_data = priv;
>  
> +		vhost_set_is_le(vq);
> +
>  		r = vhost_init_used(&n->vqs[index]);
>  
>  		mutex_unlock(&vq->mutex);
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index ad2146a9ab2d..e688eb801d43 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -113,6 +113,12 @@ static void vhost_init_is_le(struct vhost_virtqueue *vq)
>  }
>  #endif /* CONFIG_VHOST_CROSS_ENDIAN_LEGACY */
>  
> +void vhost_set_is_le(struct vhost_virtqueue *vq)
> +{
> +	vhost_init_is_le(vq);
> +}
> +EXPORT_SYMBOL_GPL(vhost_set_is_le);
> +
>  static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
>  			    poll_table *pt)
>  {

Why do we need this wrapper? Why not just export vhost_init_is_le?
Or rename vhost_init_is_le->vhost_set_is_le if you prefer.

> @@ -1156,12 +1162,8 @@ 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();
> +	if (!vq->private_data)
>  		return 0;
> -	}
> -
> -	vhost_init_is_le(vq);
>  
>  	r = vhost_update_used_flags(vq);
>  	if (r)
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index d3f767448a72..af5d33797937 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -162,6 +162,7 @@ bool vhost_enable_notify(struct vhost_dev *, struct vhost_virtqueue *);
>  
>  int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
>  		    unsigned int log_num, u64 len);
> +void vhost_set_is_le(struct vhost_virtqueue *vq);
>  
>  #define vq_err(vq, fmt, ...) do {                                  \
>  		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \