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

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Date:   Wed, 10 Jan 2018 07:15:07 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        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, 2018-01-09 at 16:39 -0800, Linus Torvalds wrote:
> 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 and smaller, no?

Not smaller, as the lfence *could* have gone in all the space left by
turning the whole retpoline thing into a single 'jmp'. But I'll
certainly give you simpler. I'll retest and merge Andi's latest patches
for that; thanks.

I'd really like to know what went wrong though. Did we merge Borislav's
attempt to peek at jumps inside alternatives, perchance? Will take a
look...
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