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:With SMAP, the 2x vhost_put_user() will also mean an extra STAC/CLAC pair,
Use one generic vhost_copy_to_user() instead of two dedicatedI don't hve a problem with this patch but do you have
accessor. This will simplify the conversion to fine grain
accessors. About 2% improvement of PPS were seen during vitio-user
Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
any idea how come removing what's supposed to be
an optimization speeds things up?
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.