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Subject: Re: [PATCH v2 2/3] x86 Test and expose CPUID faulting capabilities in
 /proc/cpuinfo
To: Dave Hansen <dave.hansen@linux.intel.com>, Kyle Huey <me@kylehuey.com>,
        "Robert O'Callahan" <robert@ocallahan.org>
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CC: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>, Borislav Petkov <bp@suse.de>,
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        <linux-kernel@vger.kernel.org>,
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        Andrew Cooper <andrew.cooper3@citrix.com>
From: David Vrabel <david.vrabel@citrix.com>
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Date: Thu, 15 Sep 2016 11:07:30 +0100
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On 14/09/16 22:35, Dave Hansen wrote:
> On 09/14/2016 02:01 PM, Kyle Huey wrote:
>> Xen advertises the underlying support for CPUID faulting but not does pass
>> through writes to the relevant MSR, nor does it virtualize it, so it does
>> not actually work. For now mask off the relevant bit on MSR_PLATFORM_INFO.
> 
> That needs to make it into a comment, please.
> 
> That *is* a Xen bug, right?

This is probably fixed in the latest version of Xen.  Andrew Cooper
would know for sure.

>> --- a/arch/x86/xen/enlighten.c
>> +++ b/arch/x86/xen/enlighten.c
>> @@ -1050,6 +1050,9 @@ static u64 xen_read_msr_safe(unsigned int msr, int *err)
>>  #endif
>>  			val &= ~X2APIC_ENABLE;
>>  		break;
>> +	case MSR_PLATFORM_INFO:
>> +		val &= ~CPUID_FAULTING_SUPPORT;
>> +		break;
>>  	}
>>  	return val;
>>  }
> 
> Does this mean that Xen guests effectively can't take advantage of this
> feature?

PV guests only.

David