linux-kernel - Re: [PATCH v2 20/39] x86/bugs: Disable Retpoline when IBT

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Message-ID: <202202241710.B35CBB06@keescook>
Date:   Thu, 24 Feb 2022 17:11:23 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, ndesaulniers@...gle.com,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 20/39] x86/bugs: Disable Retpoline when IBT

On Thu, Feb 24, 2022 at 03:51:58PM +0100, Peter Zijlstra wrote:
> Retpoline and IBT are mutually exclusive. IBT relies on indirect
> branches (JMP/CALL *%reg) while retpoline avoids them by design.
> 
> Demote to LFENCE on IBT enabled hardware.

What's the expected perf impact of this?

-Kees

-- 
Kees Cook