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

[Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>]

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?

> 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.

> 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?


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