linux-kernel - Re: Improve retpoline for Skylake

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Message-ID: <20180112220349.7dorb3lde4tffsjm@khazad-dum.debian.net>
Date:   Fri, 12 Jan 2018 20:03:49 -0200
From:   Henrique de Moraes Holschuh <hmh@....eng.br>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     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, luto@...capital.net, thomas.lendacky@....com,
        arjan.van.de.ven@...el.com
Subject: Re: Improve retpoline for Skylake

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...

However, since in this vendor-firmware-nastyness pattern the SMIs are
fired off from inside the kernel (inside the ACPI subsystem), it would
be covered by the mitigations done when entering the kernel from
syscalls and hardware interrupts.  Hopefully.

-- 
  Henrique Holschuh