linux-kernel - Re: [PATCH] x86: only use ERMS for user copies for larger sizes

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Date:   Thu, 22 Nov 2018 08:55:50 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     pabeni@...hat.com, Jens Axboe <axboe@...nel.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de,
        Peter Anvin <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>, dvlasenk@...hat.com,
        brgerst@...il.com,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes

On Thu, Nov 22, 2018 at 2:32 AM Ingo Molnar <mingo@...nel.org> wrote:
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> >
> > Random patch (with my "asm goto" hack included) attached, in case
> > people want to play with it.
>
> Doesn't even look all that hacky to me. Any hack in it that I didn't
> notice? :-)

The code to use asm goto sadly doesn't have any fallback at all for
the "no asm goto available".

I guess we're getting close to "we require asm goto support", but I
don't think we're there yet.

Also, while "unsafe_put_user()" has been converted to use asm goto
(and yes, it really does generate much nicer code), the same is not
true of "unsafe_get_user()". Because sadly, gcc does not support asm
goto with output values.

So, realistically, my patch is not _technically_ hacky, but it's
simply not viable as things stand, and it's more of a tech
demonstration than anything else.

               Linus