linux-kernel - Re: [PATCH -v2.1] x86/msr: Filter MSR writes

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Date:   Tue, 17 Nov 2020 13:21:48 -0800
From:   Matthew Garrett <mjg59@...gle.com>
To:     Mathieu Chouquet-Stringer <me@...hieu.digital>,
        Matthew Garrett <mjg59@...gle.com>,
        Chris Down <chris@...isdown.name>,
        Borislav Petkov <bp@...en8.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        sean.j.christopherson@...el.com, tony.luck@...el.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>, kernel-team@...com,
        Len Brown <lenb@...nel.org>
Subject: Re: [PATCH -v2.1] x86/msr: Filter MSR writes

On Tue, Nov 17, 2020 at 1:00 PM Mathieu Chouquet-Stringer
<me@...hieu.digital> 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.

I initially pushed back against a kernel interface for
MSR_IA32_ENERGY_PERF_BIAS (cc: Len Brown, who tried mightily to
convince me I was wrong here) on the grounds that it was exporting an
implementation detail rather than providing a generic interface, and
that it was something that could be done via userland instead. I
thought we'd end up with more examples of similar functionality and
could tie it into something more reasonable - history has proven me
wrong on that. I think it's probably reasonable to dust off the driver
that Len submitted however many years ago and push that into the
kernel now.