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

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Message-ID: <22617e57e541e460fac09db04fdb370f8e96e8ef.camel@linux.intel.com>
Date:   Tue, 08 Sep 2020 10:10:17 -0700
From:   Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     Borislav Petkov <bp@...e.de>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        kitsunyan <kitsunyan@...mail.cc>,
        "Brown, Len" <len.brown@...el.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/msr: do not warn on writes to OC_MAILBOX

On Mon, 2020-09-07 at 12:06 +0200, Borislav Petkov wrote:
> + Srinivas.
> + kitsunyan.
> 
> On Mon, Sep 07, 2020 at 11:48:43AM +0200, Jason A. Donenfeld wrote:
> > Popular tools, like intel-undervolt, use MSR 0x150 to control the
> > CPU
> > voltage offset. In fact, evidently the intel_turbo_max_3 driver in-
> > tree
> > also uses this MSR. So, teach the kernel's MSR list about this, so
> > that
> > intel-undervolt and other such tools don't spew warnings to dmesg,
> > while
> > unifying the constant used throughout the kernel.
> > 

[...]

> > -	if (reg == MSR_IA32_ENERGY_PERF_BIAS)
> > +	switch (reg) {
> > +	case MSR_IA32_ENERGY_PERF_BIAS:
There is already sysfs interface for it.

> > +	case MSR_IA32_OC_MAILBOX:
> >  		return 0;
> > +	}
> > 

[...]

> Actually, we added the filtering to catch exactly such misuses and,
> lemme check what is the proper word now... /me checks, aha, adding
> new
> MSRs to the "passlist" is the wrong thing to do.
> 
> Srinivas, can you pls convert this in-tree driver to use a proper
> sysfs
> interface for that mailbox MSR and also work with the intel-undervolt
> author - I hope I have the right person CCed from the git repo on
> github
> - to come up with a proper interface so that we can drop this MSR use
> too.

Overclocking is not architectural I/F and is supported by some special
CPU skews. I can't find any public document to specify the commands
which can be used via this OC mailbox. I have to check internally to
see if there is any. To add a proper sysfs interface we have to make
sure that we are not allowing some random commands to hardware and
crash the system.

Thanks,
Srinivas