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

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Date:   Fri, 6 Jul 2018 12:56:11 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        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>,
        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 12:31 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Jul 6, 2018 at 12:23 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> For -rc, I would favor the following simpler approach. Or I could even
>> just use get_user() instead. Thoughts ?
>
> Please just use "get_user()".
>
> In fact, we should be thinking seriosly about just removing
> __get_user() entirely. It's wrong. It optimizes the wrong thing
> entirely. It _used_ to be that the range check was noticeable, and it
> really isn't any more. These days the expensive parts are the SMAP
> costs, and both get_user() and __get_user() have those, except
> get_user() is safer and doesn't waste I$ on inlining the code to
> disable and re-enable SMAP.

If Al and Christoph ever manage to get rid of set_fs(), I bet we can
rewrite access_ok() and get_user() so that gcc can fold redundant
checks together and generate optimal code for get_user() of
consecutive struct fields all by itself.  Or maybe I'm giving gcc more
credit than it deserves.