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

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