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Subject: Re: [PATCH] prctl,x86 Add PR_[GET|SET]_CPUID for controlling the
 CPUID instruction.
To: Andy Lutomirski <luto@amacapital.net>, Kyle Huey <me@kylehuey.com>,
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From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Date: Wed, 14 Sep 2016 15:23:44 -0400
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On 09/14/2016 02:52 PM, Andy Lutomirski wrote:
> On Tue, Sep 13, 2016 at 11:13 PM, Kyle Huey <me@kylehuey.com> wrote:
>> On Mon, Sep 12, 2016 at 9:56 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>>> You should explicitly check that, if the
>>> feature is set under Xen PV, then the MSR actually works as
>>> advertised.  This may require talking to the Xen folks to make sure
>>> you're testing the right configuration.
>> This is interesting.  When running under Xen PV the kernel is allowed
>> to read the real value of MSR_PLATFORM_INFO and see that CPUID
>> faulting is supported.  But as you suggested, writing to
>> MSR_MISC_FEATURES_ENABLES doesn't actually enable CPUID faulting, at
>> least not in any way that works.
>>
>> It's not obvious to me how to test this, because when this feature
>> works, CPUID only faults in userspace, not in the kernel.  Is there
>> existing code somewhere that runs tests like this in userspace?
>>
> Andrew, Boris: should we expect Xen PV to do anything sensible when we
> write to MSR_PLATFORM_INFO to turn on CPUID faulting?  Should the Xen
> PV rdmsr hooks or perhaps the hypervisor mask out the feature if it
> isn't going to be supported?

The hypervisor uses CPUID faulting so we shouldn't advertise this
feature to guests.


-boris