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

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Message-ID: <b393a48a-6e8b-6427-373c-2825641fea99@zytor.com>
Date:   Tue, 13 Dec 2016 14:44:06 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Borislav Petkov <bp@...en8.de>,
        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 12/08/16 12:20, Borislav Petkov wrote:
> On Thu, Dec 08, 2016 at 12:08:53PM -0800, Linus Torvalds wrote:
>> Especially since that's some of the ugliest inline asm ever due to the
>> nasty BX handling.
> 
> Yeah, about that: why doesn't gcc handle that for us like it would
> handle a clobbered register? I mean, it *should* know that BX is live
> when building with -fPIC... The .ifnc thing looks really silly.
> 

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