linux-kernel - Re: [PATCH] x86/retpoline: Fix NOSPEC

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

On Tue, Jan 9, 2018 at 4:31 PM, Andi Kleen <andi@...stfloor.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