linux-kernel - Re: [RFC, PATCHv1 15/28] x86: detect 5-level paging support

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Message-ID: <20161215190902.tdle4uj27xkc3x4i@pd.tnic>
Date:   Thu, 15 Dec 2016 20:09:02 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     hpa@...or.com
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC, PATCHv1 15/28] x86: detect 5-level paging support

On Thu, Dec 15, 2016 at 09:52:12AM -0800, hpa@...or.com wrote:
> This really is only worthwhile if it ends up producing better code,
> but I doubt it.

Nah, the most it does is drops those ifnc lines in there on newer gccs.

They will appear only on
		gcc-4 and earlier and
		if we're -fPIC and
		if we're -m32 and
		if we have enough register pressure to force gcc to use the PIC	register

It was a good exercise for me to see in detail how would I go about
doing a gcc-specific workaround.

-- 
Regards/Gruss,
    Boris.

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