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

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Message-ID: <20181122112119.GA108224@gmail.com>
Date:   Thu, 22 Nov 2018 12:21:19 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.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


* Ingo Molnar <mingo@...nel.org> wrote:

> So I dug into this some more:
> 
> 1)
> 
> Firstly I tracked down GCC bloating the might_fault() checks and the 
> related out-of-line code exception handling which bloats the full 
> generated function.

Sorry, I mis-remembered that detail when I wrote the email: it was 
CONFIG_HARDENED_USERCOPY=y and its object size checks that distros enable 
- and I disabled that option to simplify the size analysis.

(might_fault() doesn't have inline conditionals so shouldn't have any 
effect on the generated code.)

Thanks,

	Ingo