linux-kernel - Re: [PATCH net-next 1/3] vhost: generalize adding used elem

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Date:   Thu, 13 Dec 2018 14:41:08 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] vhost: generalize adding used elem

On Thu, Dec 13, 2018 at 06:10:20PM +0800, Jason Wang wrote:
> Use one generic vhost_copy_to_user() instead of two dedicated
> accessor. This will simplify the conversion to fine grain accessors.
> 
> Signed-off-by: Jason Wang <jasowang@...hat.com>


The reason we did it like this is because it was faster.
Want to try benchmarking before we change it?

> ---
>  drivers/vhost/vhost.c | 11 +----------
>  1 file changed, 1 insertion(+), 10 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 3a5f81a66d34..1c54ec1b82f8 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2164,16 +2164,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
>  
>  	start = vq->last_used_idx & (vq->num - 1);
>  	used = vq->used->ring + start;
> -	if (count == 1) {
> -		if (vhost_put_user(vq, heads[0].id, &used->id)) {
> -			vq_err(vq, "Failed to write used id");
> -			return -EFAULT;
> -		}
> -		if (vhost_put_user(vq, heads[0].len, &used->len)) {
> -			vq_err(vq, "Failed to write used len");
> -			return -EFAULT;
> -		}
> -	} else if (vhost_copy_to_user(vq, used, heads, count * sizeof *used)) {
> +	if (vhost_copy_to_user(vq, used, heads, count * sizeof *used)) {
>  		vq_err(vq, "Failed to write used");
>  		return -EFAULT;
>  	}
> -- 
> 2.17.1