linux-kernel - Re: [PATCH v2 2/3] x86 Test and expose CPUID faulting capabilities in /proc/cpuinfo

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Date:   Thu, 15 Sep 2016 11:07:30 +0100
From:   David Vrabel <david.vrabel@...rix.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Kyle Huey <me@...ehuey.com>,
        Robert O'Callahan <robert@...llahan.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>, Borislav Petkov <bp@...e.de>,
        "Andy Lutomirski" <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Huang Rui" <ray.huang@....com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Len Brown <len.brown@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Aravind Gopalakrishnan <aravind.gopalakrishnan@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>,
        "moderated list:XEN HYPERVISOR INTERFACE" 
        <xen-devel@...ts.xenproject.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v2 2/3] x86 Test and expose CPUID faulting capabilities in
 /proc/cpuinfo

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