linux-kernel - Re: Improve retpoline for Skylake

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Message-Id: <DC2DB4E2-896B-4C5B-981F-4DE3EBB2EDC9@amacapital.net>
Date:   Mon, 15 Jan 2018 08:57:10 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Jon Masters <jcm@...masters.org>
Cc:     Henrique de Moraes Holschuh <hmh@....eng.br>,
        Andi Kleen <andi@...stfloor.org>,
        David Woodhouse <dwmw2@...radead.org>, tglx@...utronix.de,
        x86@...nel.org, linux-kernel@...r.kernel.org, pjt@...gle.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        peterz@...radead.org, thomas.lendacky@....com,
        arjan.van.de.ven@...el.com
Subject: Re: Improve retpoline for Skylake



> On Jan 15, 2018, at 12:26 AM, Jon Masters <jcm@...masters.org> wrote:
> 
>> On 01/12/2018 05:03 PM, Henrique de Moraes Holschuh wrote:
>> On Fri, 12 Jan 2018, Andi Kleen wrote:
>>>> Skylake still loses if it takes an SMI, right? 
>>> 
>>> SMMs are usually rare, especially on servers, and are usually
>>> not very predictible, and even if you have
>> 
>> FWIW, a data point: SMIs can be generated on demand by userspace on
>> thinkpad laptops, but they will be triggered from within a kernel
>> context.  I very much doubt this is a rare pattern...
> 
> Sure. Just touch some "legacy" hardware that the vendor emulates in a
> nasty SMI handler. It's definitely not acceptable to assume that SMIs
> can't be generated under the control of some malicious user code.
> 
> Our numbers on Skylake weren't bad, and there seem to be all kinds of
> corner cases, so again, it seems as if IBRS is the safest choice.
> 

And keep in mind that SMIs generally hit all CPUs at once, making them extra nasty.

Can we get firmware vendors to refill the return buffer just before RSM?