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Date: Wed, 10 Jan 2018 01:40:25 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
cc: Andi Kleen <andi@firstfloor.org>,
        the arch/x86 maintainers <x86@kernel.org>,
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        Dave Hansen <dave.hansen@intel.com>,
        Jiri Kosina <jikos@kernel.org>, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH] x86/retpoline: Fix NOSPEC_JMP for tip
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On Tue, 9 Jan 2018, Linus Torvalds wrote:

> On Tue, Jan 9, 2018 at 4:31 PM, Andi Kleen <andi@firstfloor.org> wrote:
> >
> > The following patch fixes it for me. Something doesn't
> > seem to work with ALTERNATIVE_2. It adds only a few bytes
> > more code, so seems acceptable.
> 
> Ugh. It's kind of stupid, though.
> 
> Why is the code sequence not simply:
> 
>   ALTERNATIVE "", "lfence", X86_FEATURE_RETPOLINE_AMD
>   ALTERNATIVE __stringify(jmp *\reg), __stringify(RETPOLINE_JMP \reg),
> X86_FEATURE_RETPOLINE
> 
> ie make that X86_FEATURE_RETPOLINE_AMD _only_ emit the "lfence", and
> simply fall through to what will be the "jmp *\reg" of the
> non-RETPOLINE version.
> 
> Then just make sure X86_FEATURE_RETPOLINE_AMD disables X86_FEATURE_RETPOLINE.
> 
> That is both simpler an dsmaller, no?

Duh, yes.