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 20:22:19 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dan Williams <dan.j.williams@...el.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 1/12/2018 7:53 PM, Dan Williams wrote:
> 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?

I don't think we need to.  On bare-metal this will be fine.  When running
as a guest, the hypervisor should have made lfence serializing, and if it
hasn't, this is still better then pause.

Thanks,
Tom

>