Subject: Re: [PATCH 3/6] vhost: switch to use vhost_add_used_n()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 16 Aug 2013 12:56:55 +0300

On Fri, Aug 16, 2013 at 01:16:27PM +0800, Jason Wang wrote:
> Let vhost_add_used() to use vhost_add_used_n() to reduce the code duplication.
> 
> Signed-off-by: Jason Wang <[email-masked]>

Does compiler inline it then?
Reason I ask, last time I checked put_user inside vhost_add_used
was much cheaper than copy_to_user inside vhost_add_used_n,
so I wouldn't be surprised if this hurt performance.
Did you check?

> ---
>  drivers/vhost/vhost.c |   43 ++-----------------------------------------
>  1 files changed, 2 insertions(+), 41 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index e58cf00..c479452 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1332,48 +1332,9 @@ EXPORT_SYMBOL_GPL(vhost_discard_vq_desc);
>   * want to notify the guest, using eventfd. */
>  int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len)
>  {
> -	struct vring_used_elem __user *used;
> +	struct vring_used_elem heads = { head, len };
>  
> -	/* The virtqueue contains a ring of used buffers.  Get a pointer to the
> -	 * next entry in that used ring. */
> -	used = &vq->used->ring[vq->last_used_idx % vq->num];
> -	if (__put_user(head, &used->id)) {
> -		vq_err(vq, "Failed to write used id");
> -		return -EFAULT;
> -	}
> -	if (__put_user(len, &used->len)) {
> -		vq_err(vq, "Failed to write used len");
> -		return -EFAULT;
> -	}
> -	/* Make sure buffer is written before we update index. */
> -	smp_wmb();
> -	if (__put_user(vq->last_used_idx + 1, &vq->used->idx)) {
> -		vq_err(vq, "Failed to increment used idx");
> -		return -EFAULT;
> -	}
> -	if (unlikely(vq->log_used)) {
> -		/* Make sure data is seen before log. */
> -		smp_wmb();
> -		/* Log used ring entry write. */
> -		log_write(vq->log_base,
> -			  vq->log_addr +
> -			   ((void __user *)used - (void __user *)vq->used),
> -			  sizeof *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);
> -	}
> -	vq->last_used_idx++;
> -	/* 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(vq->last_used_idx == vq->signalled_used))
> -		vq->signalled_used_valid = false;
> -	return 0;
> +	return vhost_add_used_n(vq, &heads, 1);
>  }
>  EXPORT_SYMBOL_GPL(vhost_add_used);
>  
> -- 
> 1.7.1