linux-kernel - Re: [PATCH v1] x86/retpoline: Use lfence in the retpoline/RSB filling RSB macros

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Date:   Fri, 12 Jan 2018 17:53:34 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     X86 ML <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jikos@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v1] x86/retpoline: Use lfence in the retpoline/RSB filling
 RSB macros

On Fri, Jan 12, 2018 at 5:07 PM, Tom Lendacky <thomas.lendacky@....com> wrote:
> The pause instruction is currently used in the retpoline and RSB filling
> macros as a speculation trap.  The use of pause was originally suggested
> because it showed a very, very small difference in the amount of
> cycles/time used to execute the retpoline as compared to lfence.  On AMD,
> the pause instruction is not a serializing instruction, so the pause/jmp
> loop will use excess power as it is speculated over waiting for return
> to mispredict to the correct target.
>
> The RSB filling macro is applicable to AMD, and, if software is unable to
> verify that lfence is serializing on AMD (possible when running under a
> hypervisor), the generic retpoline support will be used and, so, is also
> applicable to AMD.  Change the use of pause to lfence.

Should we use ASM_IFENCE for this?