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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 9 Jan 2018 16:39:19 -0800
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Subject: Re: [PATCH] x86/retpoline: Fix NOSPEC_JMP for tip
To: Andi Kleen <andi@firstfloor.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Paul Turner <pjt@google.com>,
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        Peter Zijlstra <peterz@infradead.org>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Jiri Kosina <jikos@kernel.org>, Andi Kleen <ak@linux.intel.com>
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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?

                    Linus