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

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Message-ID: <0575AF4FD06DD142AD198903C74E1CC87A5C7FE4@FMSMSX151.amr.corp.intel.com>
Date:   Sat, 13 Jan 2018 14:07:35 +0000
From:   "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
To:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>
CC:     "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>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "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

> > 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?