linux-kernel - Re: Question Regarding ERMS memcpy

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Message-ID: <C2B520E2-A4B8-4A90-BAA0-AAA44185837E@zytor.com>
Date:   Sat, 04 Mar 2017 16:56:38 -0800
From:   hpa@...or.com
To:     Borislav Petkov <bp@...e.de>
CC:     Logan Gunthorpe <logang@...tatee.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Tony Luck <tony.luck@...el.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Question Regarding ERMS memcpy

On March 4, 2017 4:33:49 PM PST, Borislav Petkov <bp@...e.de> wrote:
>On Sat, Mar 04, 2017 at 04:23:17PM -0800, hpa@...or.com wrote:
>> What are the compilation flags? It may be that gcc still does TRT
>> depending on this call site. I'd check what gcc6 or 7 generates,
>> though.
>
>Well, I don't think that matters: if you're building a kernel on one
>machine to boot on another machine, the compiler can't know at build
>time what the best MOVS* variant would be for the target machine.
>That's
>why we're doing the alternatives patching at *boot* time.
>
>However, if the size is small enough, the CALL/patch overhead would be
>definitely too much.
>
>Hmm, I wish we were able to say, "let gcc decide for small sizes and
>let
>us do the patching for larger ones."

That's what the -march= and -mtune= option do!
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