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Date: Tue, 9 Jan 2018 17:30:09 -0800
From: Andi Kleen <andi@firstfloor.org>
To: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <andi@firstfloor.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, Jan 09, 2018 at 06:45:34PM -0600, Tom Lendacky wrote:
> On 1/9/2018 6:40 PM, Thomas Gleixner wrote:
> > 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.
> 
> I think there are areas that rely on X86_FEATURE_RETPOLINE being set
> even if X86_FEATURE_RETPOLINE_AMD is set.  For example, line 261 in
> arch/x86/entry/entry_32.S is only checking for X86_FEATURE_RETPOLINE.

I audited the difference places. They all seem ok.

I assume you don't need FILL_RETURN_BUFFER on AMD. If not let me know
and we can add a X86_FEATURE_RETPOLINE_COMMON

-Andi