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

Michael S. Tsirkin
2019-01-07T14:50:19Z

On Mon, Jan 07, 2019 at 03:00:17PM +0800, Jason Wang wrote:
>
> On 2019/1/5 上午8:33, Sean Christopherson wrote:
> > 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 <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?
> > 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.
> >
>
> Yes, I don't measure, but STAC/CALC is pretty expensive when we are do very
> small copies based on the result of nosmap PPS.
>
> Thanks

Yes all this really looks like a poster child for uaccess_begin/end
plus unsafe accesses. And if these APIs don't do the job for us
then maybe better ones are needed ...

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MST