Re: [PATCH] x86/msr: do not warn on writes to OC_MAILBOX

From: Srinivas Pandruvada
Date: Tue Sep 08 2020 - 21:02:10 EST

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.
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