KVM: x86: DR6/7.RTM cannot be written

Haswell and newer Intel CPUs have support for RTM, and in that case DR6.RTM is
not fixed to 1 and DR7.RTM is not fixed to zero. That is not the case in the
current KVM implementation. This bug is apparent only if the MOV-DR instruction
is emulated or the host also debugs the guest.

This patch is a partial fix which enables DR6.RTM and DR7.RTM to be cleared and
set respectively. It also sets DR6.RTM upon every debug exception. Obviously,
it is not a complete fix, as debugging of RTM is still unsupported.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -152,14 +152,16 @@ enum {
 
 #define DR6_BD		(1 << 13)
 #define DR6_BS		(1 << 14)
-#define DR6_FIXED_1	0xffff0ff0
-#define DR6_VOLATILE	0x0000e00f
+#define DR6_RTM		(1 << 16)
+#define DR6_FIXED_1	0xfffe0ff0
+#define DR6_INIT	0xffff0ff0
+#define DR6_VOLATILE	0x0001e00f
 
 #define DR7_BP_EN_MASK	0x000000ff
 #define DR7_GE		(1 << 9)
 #define DR7_GD		(1 << 13)
 #define DR7_FIXED_1	0x00000400
-#define DR7_VOLATILE	0xffff23ff
+#define DR7_VOLATILE	0xffff2bff
 
 /* apic attention bits */
 #define KVM_APIC_CHECK_VAPIC	0

arch/x86/kvm/cpuid.h

@@ -95,4 +95,12 @@ static inline bool guest_cpuid_has_gbpages(struct kvm_vcpu *vcpu)
 	best = kvm_find_cpuid_entry(vcpu, 0x80000001, 0);
 	return best && (best->edx & bit(X86_FEATURE_GBPAGES));
 }
+
+static inline bool guest_cpuid_has_rtm(struct kvm_vcpu *vcpu)
+{
+	struct kvm_cpuid_entry2 *best;
+
+	best = kvm_find_cpuid_entry(vcpu, 7, 0);
+	return best && (best->ebx & bit(X86_FEATURE_RTM));
+}
 #endif

arch/x86/kvm/vmx.c

@@ -4892,7 +4892,7 @@ static int handle_exception(struct kvm_vcpu *vcpu)
 		if (!(vcpu->guest_debug &
 		      (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP))) {
 			vcpu->arch.dr6 &= ~15;
-			vcpu->arch.dr6 |= dr6;
+			vcpu->arch.dr6 |= dr6 | DR6_RTM;
 			if (!(dr6 & ~DR6_RESERVED)) /* icebp */
 				skip_emulated_instruction(vcpu);
 
@@ -5151,7 +5151,7 @@ static int handle_dr(struct kvm_vcpu *vcpu)
 			return 0;
 		} else {
 			vcpu->arch.dr7 &= ~DR7_GD;
-			vcpu->arch.dr6 |= DR6_BD;
+			vcpu->arch.dr6 |= DR6_BD | DR6_RTM;
 			vmcs_writel(GUEST_DR7, vcpu->arch.dr7);
 			kvm_queue_exception(vcpu, DB_VECTOR);
 			return 1;

arch/x86/kvm/x86.c

@@ -759,6 +759,15 @@ static void kvm_update_dr7(struct kvm_vcpu *vcpu)
 		vcpu->arch.switch_db_regs |= KVM_DEBUGREG_BP_ENABLED;
 }
 
+static u64 kvm_dr6_fixed(struct kvm_vcpu *vcpu)
+{
+	u64 fixed = DR6_FIXED_1;
+
+	if (!guest_cpuid_has_rtm(vcpu))
+		fixed |= DR6_RTM;
+	return fixed;
+}
+
 static int __kvm_set_dr(struct kvm_vcpu *vcpu, int dr, unsigned long val)
 {
 	switch (dr) {
@@ -774,7 +783,7 @@ static int __kvm_set_dr(struct kvm_vcpu *vcpu, int dr, unsigned long val)
 	case 6:
 		if (val & 0xffffffff00000000ULL)
 			return -1; /* #GP */
-		vcpu->arch.dr6 = (val & DR6_VOLATILE) | DR6_FIXED_1;
+		vcpu->arch.dr6 = (val & DR6_VOLATILE) | kvm_dr6_fixed(vcpu);
 		kvm_update_dr6(vcpu);
 		break;
 	case 5:
@@ -5115,7 +5124,8 @@ static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, unsigned long rflag
 	 */
 	if (unlikely(rflags & X86_EFLAGS_TF)) {
 		if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
-			kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1;
+			kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 |
+						  DR6_RTM;
 			kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip;
 			kvm_run->debug.arch.exception = DB_VECTOR;
 			kvm_run->exit_reason = KVM_EXIT_DEBUG;
@@ -5128,7 +5138,7 @@ static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, unsigned long rflag
 			 * cleared by the processor".
 			 */
 			vcpu->arch.dr6 &= ~15;
-			vcpu->arch.dr6 |= DR6_BS;
+			vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
 			kvm_queue_exception(vcpu, DB_VECTOR);
 		}
 	}
@@ -5147,7 +5157,7 @@ static bool kvm_vcpu_check_breakpoint(struct kvm_vcpu *vcpu, int *r)
 					   vcpu->arch.eff_db);
 
 		if (dr6 != 0) {
-			kvm_run->debug.arch.dr6 = dr6 | DR6_FIXED_1;
+			kvm_run->debug.arch.dr6 = dr6 | DR6_FIXED_1 | DR6_RTM;
 			kvm_run->debug.arch.pc = kvm_rip_read(vcpu) +
 				get_segment_base(vcpu, VCPU_SREG_CS);
 
@@ -5166,7 +5176,7 @@ static bool kvm_vcpu_check_breakpoint(struct kvm_vcpu *vcpu, int *r)
 
 		if (dr6 != 0) {
 			vcpu->arch.dr6 &= ~15;
-			vcpu->arch.dr6 |= dr6;
+			vcpu->arch.dr6 |= dr6 | DR6_RTM;
 			kvm_queue_exception(vcpu, DB_VECTOR);
 			*r = EMULATE_DONE;
 			return true;
@@ -6866,7 +6876,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu)
 	kvm_clear_exception_queue(vcpu);
 
 	memset(vcpu->arch.db, 0, sizeof(vcpu->arch.db));
-	vcpu->arch.dr6 = DR6_FIXED_1;
+	vcpu->arch.dr6 = DR6_INIT;
 	kvm_update_dr6(vcpu);
 	vcpu->arch.dr7 = DR7_FIXED_1;
 	kvm_update_dr7(vcpu);