Re: [PATCH] x86: avoid #GP for PV guest MSR accesses

[Sergey Dyasli <sergey.dyasli@citrix.com>]

On Fri, 2017-09-22 at 03:06 -0600, Jan Beulich wrote:
> Halfway recent Linux kernels probe MISC_FEATURES_ENABLES on all CPUs,
> leading to ugly recovered #GP fault messages with debug builds on older
> systems. We can do better, so introduce synthetic feature flags for
> both this and PLATFORM_INFO to avoid the rdmsr_safe() altogether.
> 
> The rdmsr_safe() uses for MISC_ENABLE are left in place as benign - it
> exists for all 64-bit capable Intel CPUs (see e.g. early_init_intel()).
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

The intent of this patch (and the related "VMX: PLATFORM_INFO MSR is
r/o") is somewhat intersects with my series "Generic MSR policy:
infrastructure + cpuid_faulting". IMHO it's better to fix MSR-related
issues in the scope of the MSR policy work.

Also, I have one question below.

> 
> --- a/xen/arch/x86/cpu/intel.c
> +++ b/xen/arch/x86/cpu/intel.c
> @@ -21,10 +21,19 @@ static bool __init probe_intel_cpuid_fau
>  {
>  	uint64_t x;
>  
> -	if (rdmsr_safe(MSR_INTEL_PLATFORM_INFO, x) ||
> -	    !(x & MSR_PLATFORM_INFO_CPUID_FAULTING))
> +	if (rdmsr_safe(MSR_INTEL_PLATFORM_INFO, x))
>  		return 0;
>  
> +	setup_force_cpu_cap(X86_FEATURE_MSR_PLATFORM_INFO);
> +
> +	if (!(x & MSR_PLATFORM_INFO_CPUID_FAULTING)) {
> +		if (!rdmsr_safe(MSR_INTEL_MISC_FEATURES_ENABLES, x))
> +			setup_force_cpu_cap(X86_FEATURE_MSR_MISC_FEATURES);
> +		return 0;
> +	}
> +
> +	setup_force_cpu_cap(X86_FEATURE_MSR_MISC_FEATURES);
> +
>  	expected_levelling_cap |= LCAP_faulting;
>  	levelling_caps |=  LCAP_faulting;
>  	setup_force_cpu_cap(X86_FEATURE_CPUID_FAULTING);
> --- a/xen/arch/x86/pv/emul-priv-op.c
> +++ b/xen/arch/x86/pv/emul-priv-op.c
> @@ -941,8 +941,7 @@ static int read_msr(unsigned int reg, ui
>          return X86EMUL_OKAY;
>  
>      case MSR_INTEL_PLATFORM_INFO:
> -        if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
> -             rdmsr_safe(MSR_INTEL_PLATFORM_INFO, *val) )
> +        if ( !boot_cpu_has(X86_FEATURE_MSR_PLATFORM_INFO) )
>              break;
>          *val = 0;
>          if ( this_cpu(cpuid_faulting_enabled) )
> @@ -950,8 +949,7 @@ static int read_msr(unsigned int reg, ui
>          return X86EMUL_OKAY;
>  
>      case MSR_INTEL_MISC_FEATURES_ENABLES:
> -        if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
> -             rdmsr_safe(MSR_INTEL_MISC_FEATURES_ENABLES, *val) )
> +        if ( !boot_cpu_has(X86_FEATURE_MSR_MISC_FEATURES) )
>              break;
>          *val = 0;
>          if ( curr->arch.cpuid_faulting )
> @@ -1147,15 +1145,13 @@ static int write_msr(unsigned int reg, u
>          return X86EMUL_OKAY;
>  
>      case MSR_INTEL_PLATFORM_INFO:
> -        if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
> -             val || rdmsr_safe(MSR_INTEL_PLATFORM_INFO, val) )
> +        if ( !boot_cpu_has(X86_FEATURE_MSR_PLATFORM_INFO) || val )
>              break;
>          return X86EMUL_OKAY;

Why writes to MSR_INTEL_PLATFORM_INFO shouldn't produce #GP faults for
PV guests?

>  
>      case MSR_INTEL_MISC_FEATURES_ENABLES:
> -        if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
> -             (val & ~MSR_MISC_FEATURES_CPUID_FAULTING) ||
> -             rdmsr_safe(MSR_INTEL_MISC_FEATURES_ENABLES, temp) )
> +        if ( !boot_cpu_has(X86_FEATURE_MSR_MISC_FEATURES) ||
> +             (val & ~MSR_MISC_FEATURES_CPUID_FAULTING) )
>              break;
>          if ( (val & MSR_MISC_FEATURES_CPUID_FAULTING) &&
>               !this_cpu(cpuid_faulting_enabled) )
> --- a/xen/include/asm-x86/cpufeatures.h
> +++ b/xen/include/asm-x86/cpufeatures.h
> @@ -22,3 +22,5 @@ XEN_CPUFEATURE(APERFMPERF,      (FSCAPIN
>  XEN_CPUFEATURE(MFENCE_RDTSC,    (FSCAPINTS+0)*32+ 9) /* MFENCE synchronizes RDTSC */
>  XEN_CPUFEATURE(XEN_SMEP,        (FSCAPINTS+0)*32+10) /* SMEP gets used by Xen itself */
>  XEN_CPUFEATURE(XEN_SMAP,        (FSCAPINTS+0)*32+11) /* SMAP gets used by Xen itself */
> +XEN_CPUFEATURE(MSR_PLATFORM_INFO, (FSCAPINTS+0)*32+12) /* PLATFORM_INFO MSR present */
> +XEN_CPUFEATURE(MSR_MISC_FEATURES, (FSCAPINTS+0)*32+13) /* MISC_FEATURES_ENABLES MSR present */
> 
> 
> 
> 
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-- 
Thanks,
Sergey
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