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Subject: Re: [PATCH] x86: static_cpu_has_safe: discard dynamic check after init
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Wed, 20 Jan 2016 02:55:24 -0800
To: Borislav Petkov <bp@suse.de>
CC: Brian Gerst <brgerst@gmail.com>, the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Denys Vlasenko <dvlasenk@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>
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On January 20, 2016 2:50:18 AM PST, Borislav Petkov <bp@suse.de> wrote:
>On Tue, Jan 19, 2016 at 08:42:10PM -0800, H. Peter Anvin wrote:
>> Right.  I wish we could tag reference call sites as clean, not
>sources
>> or targets.  Sigh.
>
>I was thinking about something like this:
>
>From: Borislav Petkov <bp@suse.de>
>Date: Wed, 20 Jan 2016 11:45:36 +0100
>Subject: [PATCH] x86/alternatives: Add an auxilary section
>
>Add .altinstr_aux for additional instructions which will be used before
>and/or during patching. All stuff which needs more sophisticated
>patching should go there. See next patch.
>
>Signed-off-by: Borislav Petkov <bp@suse.de>
>---
> arch/x86/kernel/vmlinux.lds.S | 5 +++++
> 1 file changed, 5 insertions(+)
>
>diff --git a/arch/x86/kernel/vmlinux.lds.S
>b/arch/x86/kernel/vmlinux.lds.S
>index 74e4bf11f562..35868bf529b9 100644
>--- a/arch/x86/kernel/vmlinux.lds.S
>+++ b/arch/x86/kernel/vmlinux.lds.S
>@@ -244,6 +244,11 @@ SECTIONS
> 	 */
>	.altinstr_replacement : AT(ADDR(.altinstr_replacement) - LOAD_OFFSET)
>{
> 		*(.altinstr_replacement)
>+		/*
>+		 * Auxiliary section for misc instruction patching tasks. See
>+		 * static_cpu_has(), for an example.
>+		 */
>+		*(.altinstr_aux)
> 	}
> 
> 	/*

How about:

section for code used exclusively before alternatives are run.  All references to such code must be patched out by alternatives, normally by using a patch with X86_FEATURE_ALWAYS.

See static_cpu_has() for an example.
-- 
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