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

[Jason Wang <jasowang@redhat.com>]

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