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Date:   Tue, 17 Nov 2020 22:20:16 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Mathieu Chouquet-Stringer <me@mathieu.digital>
Cc:     Matthew Garrett <mjg59@google.com>,
        Chris Down <chris@chrisdown.name>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        sean.j.christopherson@intel.com, tony.luck@intel.com,
        Linus Torvalds <torvalds@linux-foundation.org>,
        the arch/x86 maintainers <x86@kernel.org>, kernel-team@fb.com
Subject: Re: [PATCH -v2.1] x86/msr: Filter MSR writes
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On Tue, Nov 17, 2020 at 10:00:18PM +0100, Mathieu Chouquet-Stringer wrote:
> I'm late to the party but it seems allowing MSR_IA32_ENERGY_PERF_BIAS
> has the downside of flagging the kernel as tainted without telling you
> why if you use something like x86_energy_perf_policy (from
> tools/power/x86/x86_energy_perf_policy) which itself is used by tuned.

Not for long:

https://git.kernel.org/tip/fe0a5788624c8b8f113a35bbe4636e37f9321241

> So while both documentation and tools should be updated as to be clearer
> and to not taint the kernel respectively, there's something that remains
> to be done to explain why or how the kernel got tainted because of
> poking into MSRs...

Because if you poke at random MSRs and you manage to "configure" your
CPU to run "out of spec" - this is what the taint flag is called:
TAINT_CPU_OUT_OF_SPEC - then this is exactly the case you've created: a
CPU executing outside of specifications.

I agree with the update-the-documentation aspect - S does not mean only
SMP kernel on !SMP-capable CPU but the more general, CPU is out of spec.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette