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

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Date:   Wed, 14 Dec 2016 00:07:54 +0100
From:   Boris Petkov <bp@...en8.de>
To:     "H. Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     "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 December 13, 2016 11:44:06 PM GMT+01:00, "H. Peter Anvin" <hpa@...or.com> wrote:
>When compiling with -fPIC gcc treats ebx as a "fixed register".  A
>fixed
>register can't be spilled, and so a clobber of a fixed register is a
>fatal error.
>
>Like it or not, it's how it works.
>
>	-hpa

In the meantime I talked to my gcc guy and here's the deal:

There are gcc versions (4.x and earlier) which do not save/restore the PIC register around an inline asm even if it is one of the registers that the inline asm clobbers. Therefore the saving/restoring needs to be done by the inline asm itself.

5.x and later handle that fine.

Thus I was thinking of adding a build-time check for the gcc version but that might turn out to be more code in the end than those ugly ifnc clauses. 
-- 
Sent from a small device: formatting sux and brevity is inevitable.