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Date:   Fri, 12 Jun 2020 22:39:35 +0200
From:   Peter Zijlstra <>
To:     Borislav Petkov <>
Cc:     Linus Torvalds <>,
        x86-ml <>, lkml <>
Subject: Re: [RFC PATCH] x86/msr: Filter MSR writes

On Fri, Jun 12, 2020 at 07:48:01PM +0200, Borislav Petkov wrote:
> On Fri, Jun 12, 2020 at 10:20:03AM -0700, Linus Torvalds wrote:
> > Since you already added the filtering, this looks fairly sane.
> > 
> > IOW, what MSR's do we expect people to maybe write to normally? You
> > added MSR_IA32_ENERGY_PERF_BIAS as an allowed MST, maybe there are
> > others?
> Right, this MSR is being written by cpupower in tools/. My search was
> confined within the kernel source only so there very likely are others.

So that tool writing to /dev/msr has already caused pain; the direct
result is that the intel pstate driver doesn't want to use an MSR shadow
variable to avoid RDMSR because that'd loose input.

(sorry, that's what google found me)

So ideally we'd just disallow it too. It already has a sysfs file (per
those patches):


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