linux-kernel - Re: Question Regarding ERMS memcpy

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Date:   Sun, 5 Mar 2017 01:33:49 +0100
From:   Borislav Petkov <bp@...e.de>
To:     hpa@...or.com
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 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."

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--