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

From: Sean Christopherson <[email-masked]>
Date: Fri, 4 Jan 2019 16:33:39 -0800

On Fri, Jan 04, 2019 at 04:29:34PM -0500, Michael S. Tsirkin wrote:
> 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 <[email-masked]>
> 
> 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?

With SMAP, the 2x vhost_put_user() will also mean an extra STAC/CLAC pair,
which is probably slower than the overhead of CALL+RET to whatever flavor
of copy_user_generic() gets used.  CALL+RET is really the only overhead
since all variants of copy_user_generic() unroll accesses smaller than
64 bytes, e.g. on a 64-bit system, __copy_to_user() will write all 8
bytes in a single MOV.

Removing the special casing also eliminates a few hundred bytes of code
as well as the need for hardware to predict count==1 vs. count>1.

> 
> > ---
> >  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