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

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Date:   Sat, 13 Jan 2018 22:36:13 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Tom Lendacky <thomas.lendacky@....com>
cc:     "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "riel@...hat.com" <riel@...hat.com>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "pjt@...gle.com" <pjt@...gle.com>, "bp@...en8.de" <bp@...en8.de>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v1] x86/retpoline: Use lfence in the retpoline/RSB filling
 RSB macros

On Sat, 13 Jan 2018, Tom Lendacky wrote:

> On 1/13/2018 8:07 AM, Van De Ven, Arjan wrote:
> >>> 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.
> >>>
> >>> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> >>
> >> Conditionally-Acked-by: David Woodhouse <dwmw@...zon.co.uk>
> > 
> > 
> > pause is technically the "save me power" instruction
> > 
> > how about a compromise where we do a double:
> > 
> > pause
> > lfence
> > jmp <up>
> > 
> > as sequence... that way if the branch recovery is fast, we get the performance of pause, but if it takes a while, on AMD you get the behavior of lfence?
> 
> That should work on AMD.

I zapped the commit from tip for now until this discussion is resolved.

Thanks,

	tglx