Re: [RFC PATCH for 4.18 3/5] rseq: uapi: declare rseq_cs field as union, update includes

On Fri, Jul 6, 2018 at 12:38 PM Mathieu Desnoyers
[off-list ref] wrote:

Should I change all 4 bytes __get_user()/__put_user() in kernel/rseq.c
for get_user()/put_user() to ensure consistency ?

Probably.

*If* this actually turns out to be somethinig that shows up on
profiles, it's almost certainly going to be the STAC/CLAC instructions
("perf report" tends to report them as three one-byte nop's because
that's how they look before instruction replacement).

And then it's not __get/put_user() that will improve things, but doing a

        user_access_begin();

        .. do unsafe_get/put_user() ..

        user_access_end();

that will improve performance.

But it is *very* seldom useful. We have it in a handful of places in
the kernel, and the most noticeable one is
lib/{strnlen,strncpy_from}_user.c

                       Linus