Re: [RFC PATCH V3 1/5] vhost: generalize adding used elem

Michael S. Tsirkin
2019-01-04T21:29:34Z

On Sat, Dec 29, 2018 at 08:46:52PM +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. About 2% improvement of PPS were seen during vitio-user
> txonly test.
>
> Signed-off-by: Jason Wang <jasowang@redhat.com>

I don't hve a problem with this patch but do you have
any idea how come removing what's supposed to be
an optimization speeds things up?

> ---
> 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 55e5aa662ad5..f179b5ee14c4 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2174,16 +2174,7 @@ 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