x86/Intel: hide CPUID faulting capability from guests

We don't currently emulate it, so guests should not be misguided to
believe they can (try to) use it.

For now, simply return zero to guests for platform MSR reads, and only
accept (by discarding) writes of zero. If ever there will be bits we
can safely expose to guests, let's handle them by white listing.

(As a side note - according to SDM version 059 bit 31 is reserved on
all known families.)

Reported-by: Kyle Huey <me@xxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2699,6 +2699,13 @@ static int vmx_msr_read_intercept(unsign
         if ( vpmu_do_rdmsr(msr, msr_content) )
             goto gp_fault;
         break;
+
+    case MSR_INTEL_PLATFORM_INFO:
+        if ( rdmsr_safe(MSR_INTEL_PLATFORM_INFO, *msr_content) )
+            goto gp_fault;
+        *msr_content = 0;
+        break;
+
     default:
         if ( passive_domain_do_rdmsr(msr, msr_content) )
             goto done;
@@ -2918,6 +2925,13 @@ static int vmx_msr_write_intercept(unsig
          if ( vpmu_do_wrmsr(msr, msr_content, 0) )
             goto gp_fault;
         break;
+
+    case MSR_INTEL_PLATFORM_INFO:
+        if ( msr_content ||
+             rdmsr_safe(MSR_INTEL_PLATFORM_INFO, msr_content) )
+            goto gp_fault;
+        break;
+
     default:
         if ( passive_domain_do_wrmsr(msr, msr_content) )
             return X86EMUL_OKAY;
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2938,6 +2938,14 @@ static int emulate_privileged_op(struct
             if ( v->arch.debugreg[7] & DR7_ACTIVE_MASK )
                 wrmsrl(regs->_ecx, msr_content);
             break;
+
+        case MSR_INTEL_PLATFORM_INFO:
+            if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
+                 msr_content ||
+                 rdmsr_safe(MSR_INTEL_PLATFORM_INFO, msr_content) )
+                goto fail;
+            break;
+
         case MSR_P6_PERFCTR(0)...MSR_P6_PERFCTR(7):
         case MSR_P6_EVNTSEL(0)...MSR_P6_EVNTSEL(3):
         case MSR_CORE_PERF_FIXED_CTR0...MSR_CORE_PERF_FIXED_CTR2:
@@ -3066,6 +3074,14 @@ static int emulate_privileged_op(struct
             /* No extra capabilities are supported */
             regs->eax = regs->edx = 0;
             break;
+
+        case MSR_INTEL_PLATFORM_INFO:
+            if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
+                 rdmsr_safe(MSR_INTEL_PLATFORM_INFO, val) )
+                goto fail;
+            regs->eax = regs->edx = 0;
+            break;
+
         case MSR_P6_PERFCTR(0)...MSR_P6_PERFCTR(7):
         case MSR_P6_EVNTSEL(0)...MSR_P6_EVNTSEL(3):
         case MSR_CORE_PERF_FIXED_CTR0...MSR_CORE_PERF_FIXED_CTR2: