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

[Kyle Huey <me@xxxxxxxxxxxx>]

On Wed, Sep 14, 2016 at 2:35 PM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> 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?

Yes.  Xen needs to either not advertise the feature or actually
support it.  This came up in the prior thread ("[PATCH] prctl,x86 Add
PR_[GET|SET]_CPUID for controlling the CPUID instruction.").

>> Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>
>> ---
>>  arch/x86/include/asm/cpufeatures.h |  1 +
>>  arch/x86/include/asm/msr-index.h   |  1 +
>>  arch/x86/kernel/cpu/scattered.c    | 14 ++++++++++++++
>>  arch/x86/xen/enlighten.c           |  3 +++
>>  4 files changed, 19 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/cpufeatures.h 
>> b/arch/x86/include/asm/cpufeatures.h
>> index 92a8308..78b9d06 100644
>> --- a/arch/x86/include/asm/cpufeatures.h
>> +++ b/arch/x86/include/asm/cpufeatures.h
>> @@ -190,6 +190,7 @@
>>
>>  #define X86_FEATURE_CPB              ( 7*32+ 2) /* AMD Core Performance 
>> Boost */
>>  #define X86_FEATURE_EPB              ( 7*32+ 3) /* IA32_ENERGY_PERF_BIAS 
>> support */
>> +#define X86_FEATURE_CPUID_FAULT ( 7*32+ 4) /* Intel CPUID faulting */
>>
>>  #define X86_FEATURE_HW_PSTATE        ( 7*32+ 8) /* AMD HW-PState */
>>  #define X86_FEATURE_PROC_FEEDBACK ( 7*32+ 9) /* AMD ProcFeedbackInterface */
>> diff --git a/arch/x86/include/asm/msr-index.h 
>> b/arch/x86/include/asm/msr-index.h
>> index 56f4c66..83908d5 100644
>> --- a/arch/x86/include/asm/msr-index.h
>> +++ b/arch/x86/include/asm/msr-index.h
>> @@ -41,6 +41,7 @@
>>  #define MSR_IA32_PERFCTR1            0x000000c2
>>  #define MSR_FSB_FREQ                 0x000000cd
>>  #define MSR_PLATFORM_INFO            0x000000ce
>> +#define CPUID_FAULTING_SUPPORT               (1UL << 31)
>>
>>  #define MSR_NHM_SNB_PKG_CST_CFG_CTL  0x000000e2
>>  #define NHM_C3_AUTO_DEMOTE           (1UL << 25)
>> diff --git a/arch/x86/kernel/cpu/scattered.c 
>> b/arch/x86/kernel/cpu/scattered.c
>> index 8cb57df..d502da1 100644
>> --- a/arch/x86/kernel/cpu/scattered.c
>> +++ b/arch/x86/kernel/cpu/scattered.c
>> @@ -24,6 +24,17 @@ enum cpuid_regs {
>>       CR_EBX
>>  };
>>
>> +static int supports_cpuid_faulting(void)
>> +{
>> +     unsigned int lo, hi;
>> +
>> +     if (rdmsr_safe(MSR_PLATFORM_INFO, &lo, &hi) == 0 &&
>> +         (lo & CPUID_FAULTING_SUPPORT))
>> +             return 1;
>> +     else
>> +             return 0;
>> +}
>
> Is any of this useful to optimize away at compile-time?  We have config
> options for when we're running as a guest, and this seems like a feature
> that isn't available when running on bare metal.

On the contrary, this is only available when we're on bare metal.
Neither Xen nor KVM virtualize CPUID faulting (although KVM correctly
suppresses MSR_PLATFORM_INFO's report of support for it).

>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
>> index b86ebb1..2c47f0c 100644
>> --- 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?

Yes.

- Kyle

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