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SubjectRe: [PATCH v2 2/3] x86 Test and expose CPUID faulting capabilities in /proc/cpuinfo
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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@kylehuey.com>
> ---
> 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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