[PATCH] CPUID Masking MSRs

[Alexander Graf <agraf@suse.de>]

Current AMD CPUs support masking of CPUID bits. Using this functionality,
a VMM can limit what features are exposed to the guest, even if it's not
using SVM/VMX.

While I'm not aware of any open source hypervisor that uses these MSRs
atm, VMware ESX does and patches exist for Xen, where trapping CPUID is
non-trivial.

This patch implements emulation for this masking, which is pretty trivial
because we're intercepting CPUID anyways.

Because it's so simple and can be pretty effective, I put it into the
generic code paths, so VMX benefits from it as well.

Signed-off-by: Alexander Graf <agraf@suse.de>

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 863ea73..e2f0dde 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -370,6 +370,9 @@ struct kvm_vcpu_arch {
 	unsigned long dr6;
 	unsigned long dr7;
 	unsigned long eff_db[KVM_NR_DB_REGS];
+
+	u64 cpuid_mask;
+	u64 cpuid_mask_ext;
 };
 
 struct kvm_mem_alias {
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 1890032..03b53ba 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -337,5 +337,7 @@
 
 #define MSR_VM_CR                       0xc0010114
 #define MSR_VM_HSAVE_PA                 0xc0010117
+#define MSR_VM_MASK_CPUID               0xc0011004
+#define MSR_VM_MASK_CPUID_EXT           0xc0011005
 
 #endif /* _ASM_X86_MSR_INDEX_H */
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 18bba94..83b4877 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -782,6 +784,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
 		kvm_write_guest_time(vcpu);
 		break;
 	}
+	case MSR_VM_MASK_CPUID:
+		vcpu->arch.cpuid_mask = data;
+		break;
+	case MSR_VM_MASK_CPUID_EXT:
+		vcpu->arch.cpuid_mask_ext = data;
+		break;
 	default:
 		pr_unimpl(vcpu, "unhandled wrmsr: 0x%x data %llx\n", msr, data);
 		return 1;
@@ -896,6 +904,12 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
 	case MSR_KVM_SYSTEM_TIME:
 		data = vcpu->arch.time;
 		break;
+	case MSR_VM_MASK_CPUID:
+		data = vcpu->arch.cpuid_mask;
+		break;
+	case MSR_VM_MASK_CPUID_EXT:
+		data = vcpu->arch.cpuid_mask_ext;
+		break;
 	default:
 		pr_unimpl(vcpu, "unhandled rdmsr: 0x%x\n", msr);
 		return 1;
@@ -2901,10 +2915,19 @@ void kvm_emulate_cpuid(struct kvm_vcpu *vcpu)
 	kvm_register_write(vcpu, VCPU_REGS_RDX, 0);
 	best = kvm_find_cpuid_entry(vcpu, function, index);
 	if (best) {
+		u32 ecx = best->ecx;
+		u32 edx = best->edx;
 		kvm_register_write(vcpu, VCPU_REGS_RAX, best->eax);
 		kvm_register_write(vcpu, VCPU_REGS_RBX, best->ebx);
-		kvm_register_write(vcpu, VCPU_REGS_RCX, best->ecx);
-		kvm_register_write(vcpu, VCPU_REGS_RDX, best->edx);
+		if ( function == 1 ) {
+			ecx &= (u32)vcpu->arch.cpuid_mask;
+			edx &= (u32)(vcpu->arch.cpuid_mask >> 32);
+		} else if ( function == 0x80000001 ) {
+			ecx &= (u32)vcpu->arch.cpuid_mask_ext;
+			edx &= (u32)(vcpu->arch.cpuid_mask_ext >> 32);
+		}
+		kvm_register_write(vcpu, VCPU_REGS_RCX, ecx);
+		kvm_register_write(vcpu, VCPU_REGS_RDX, edx);
 	}
 	kvm_x86_ops->skip_emulated_instruction(vcpu);
 	KVMTRACE_5D(CPUID, vcpu, function,
@@ -4089,6 +4112,8 @@ int kvm_arch_vcpu_reset(struct kvm_vcpu *vcpu)
 	memset(vcpu->arch.db, 0, sizeof(vcpu->arch.db));
 	vcpu->arch.dr6 = DR6_FIXED_1;
 	vcpu->arch.dr7 = DR7_FIXED_1;
+	vcpu->arch.cpuid_mask = 0xffffffffffffffff;
+	vcpu->arch.cpuid_mask_ext = 0xffffffffffffffff;
 
 	return kvm_x86_ops->vcpu_reset(vcpu);
 }
-- 
1.5.6