linux-kernel - Re: [RFC PATCH for 4.18 3/5] rseq: uapi: declare rseq

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Date:   Fri, 6 Jul 2018 12:56:58 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King - ARM Linux <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
        Andi Kleen <andi@...stfloor.org>,
        Christoph Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: 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
<mathieu.desnoyers@...icios.com> 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