linux-kernel - Re: [PATCH] x86/msr: do not warn on writes to OC

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Date:   Tue, 08 Sep 2020 18:02:05 -0700
From:   Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>,
        Sultan Alsawaf <sultan@...neltoast.com>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        kitsunyan <kitsunyan@...mail.cc>,
        "Brown, Len" <len.brown@...el.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/msr: do not warn on writes to OC_MAILBOX

On Tue, 2020-09-08 at 21:30 +0200, Borislav Petkov wrote:
> On Tue, Sep 08, 2020 at 12:18:38PM -0700, Sultan Alsawaf wrote:
> > I'd like to point out that on Intel's recent 14nm parts,
> > undervolting
> > is not so much for squeezing every last drop of performance out of
> > the
> > SoC as it is for necessity.
> 
> <snip interesting examples>
> 
> Sounds to me that this undervolting functionality should be part of
> the kernel and happen automatically. I have no clue, though, whether
> people who do it, just get lucky and undervolting doesn't cause any
> other hardware issues, or there's a real reason for this power
> madness
> and if not done, power-related failures happen only on some boxes so
> they decided to do them on all.
> 
> Or maybe BIOS is nuts, which is not a stretch.
> 
The whole OC is based on experiments to come to correct values. This
depends on whole system design not just CPUs.
https://www.intel.com/content/www/us/en/gaming/resources/how-to-overclock.html
It warns about system stability.

> Srinivas, what's the story here?
I checked and there is no public spec. There are several mailbox
commands with version dependent on the processor.

The actual OC mailbox implementation itself is implemented in Linux in
intel_turbo_max_3 driver. So that is public.
So someone can develop a driver and provide some sysfs to send mailbox
commands, but kernel can't validate commands which can cause any
security or stability issues. Not sure if this is acceptable standard.
I don't think there is any precedent of creating such blind sysfs
entries.


Thanks,
Srinivas