Subject: Re: [PATCH net-next RFC 3/5] vhost: introduce vhost_add_used_idx()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 26 Sep 2017 22:13:09 +0300

On Fri, Sep 22, 2017 at 04:02:33PM +0800, Jason Wang wrote:
> This patch introduces a helper which just increase the used idx. This
> will be used in pair with vhost_prefetch_desc_indices() by batching
> code.
> 
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/vhost/vhost.c | 33 +++++++++++++++++++++++++++++++++
>  drivers/vhost/vhost.h |  1 +
>  2 files changed, 34 insertions(+)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 8424166d..6532cda 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2178,6 +2178,39 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len)
>  }
>  EXPORT_SYMBOL_GPL(vhost_add_used);
>  
> +int vhost_add_used_idx(struct vhost_virtqueue *vq, int n)
> +{
> +	u16 old, new;
> +
> +	old = vq->last_used_idx;
> +	new = (vq->last_used_idx += n);
> +	/* If the driver never bothers to signal in a very long while,
> +	 * used index might wrap around. If that happens, invalidate
> +	 * signalled_used index we stored. TODO: make sure driver
> +	 * signals at least once in 2^16 and remove this.
> +	 */
> +	if (unlikely((u16)(new - vq->signalled_used) < (u16)(new - old)))
> +		vq->signalled_used_valid = false;
> +
> +	/* Make sure buffer is written before we update index. */
> +	smp_wmb();
> +	if (vhost_put_user(vq, cpu_to_vhost16(vq, vq->last_used_idx),
> +			   &vq->used->idx)) {
> +		vq_err(vq, "Failed to increment used idx");
> +		return -EFAULT;
> +	}
> +	if (unlikely(vq->log_used)) {
> +		/* Log used index update. */
> +		log_write(vq->log_base,
> +			  vq->log_addr + offsetof(struct vring_used, idx),
> +			  sizeof(vq->used->idx));
> +		if (vq->log_ctx)
> +			eventfd_signal(vq->log_ctx, 1);
> +	}
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(vhost_add_used_idx);
> +
>  static int __vhost_add_used_n(struct vhost_virtqueue *vq,
>  			    struct vring_used_elem *heads,
>  			    unsigned count)
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 16c2cb6..5dd6c05 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -199,6 +199,7 @@ int __vhost_get_vq_desc(struct vhost_virtqueue *vq,
>  void vhost_discard_vq_desc(struct vhost_virtqueue *, int n);
>  
>  int vhost_vq_init_access(struct vhost_virtqueue *);
> +int vhost_add_used_idx(struct vhost_virtqueue *vq, int n);
>  int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
>  int vhost_add_used_n(struct vhost_virtqueue *, struct vring_used_elem *heads,
>  		     unsigned count);

Please change the API to hide the fact that there's an index that needs
to be updated.

> -- 
> 2.7.4