Messages in this thread | | | Subject | Re: [RFC PATCH V3 1/5] vhost: generalize adding used elem | From | Jason Wang <> | Date | Mon, 7 Jan 2019 15:00:17 +0800 |
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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
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