[Xen-devel] [PATCH for-4.9 v2] x86: avoid #GP for PV guest MSR accesses

["Jan Beulich" <JBeulich@xxxxxxxx>]

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.

Note that the r/o nature of PLATFORM_INFO is now also being enforced.

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@xxxxxxxx>
---
v2: Force PLATFORM_INFO writes to fail.

--- 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/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2626,8 +2626,7 @@ 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) )
@@ -2635,8 +2634,7 @@ 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 )
@@ -2834,15 +2832,12 @@ 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) )
-            break;
-        return X86EMUL_OKAY;
+        /* The MSR is read-only. */
+        break;
 
     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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