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

From: Jason Wang
Date: Mon Jan 07 2019 - 02:00:32 EST

On 2019/1/5 äå8:33, 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@xxxxxxxxxx>
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.