[RFC PATCH v3 18/59] KVM: x86: Add flag to mark TSC as immutable (for TDX)

[isaku . yamahata]

From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

The TSC for TDX1 guests is fixed at TD creation time.  Add tsc_immutable
to reflect that the TSC of the guest cannot be changed in any way, and
use it to short circuit all paths that lead to one of the myriad TSC
adjustment flows.

Suggested-by: Kai Huang <kai.huang@xxxxxxxxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/x86.c              | 35 +++++++++++++++++++++++++--------
 2 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e912e1e853ef..f3808672c720 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1122,6 +1122,7 @@ struct kvm_arch {
 	int audit_point;
 	#endif
 
+	bool tsc_immutable;
 	bool backwards_tsc_observed;
 	bool boot_vcpu_runs_old_kvmclock;
 	u32 bsp_vcpu_id;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fefa4602e879..0ebd60846079 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2247,7 +2247,9 @@ static int set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz, bool scale)
 	u64 ratio;
 
 	/* Guest TSC same frequency as host TSC? */
-	if (!scale) {
+	if (!scale || vcpu->kvm->arch.tsc_immutable) {
+		if (scale)
+			pr_warn_ratelimited("Guest TSC immutable, scaling not supported\n");
 		kvm_vcpu_write_tsc_multiplier(vcpu, kvm_default_tsc_scaling_ratio);
 		return 0;
 	}
@@ -2534,6 +2536,9 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
 	bool matched = false;
 	bool synchronizing = false;
 
+	if (WARN_ON_ONCE(vcpu->kvm->arch.tsc_immutable))
+		return;
+
 	raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
 	offset = kvm_compute_l1_tsc_offset(vcpu, data);
 	ns = get_kvmclock_base_ns();
@@ -2960,6 +2965,10 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	u8 pvclock_flags;
 	bool use_master_clock;
 
+	/* Unable to update guest time if the TSC is immutable. */
+	if (ka->tsc_immutable)
+		return 0;
+
 	kernel_ns = 0;
 	host_tsc = 0;
 
@@ -4372,7 +4381,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		if (tsc_delta < 0)
 			mark_tsc_unstable("KVM discovered backwards TSC");
 
-		if (kvm_check_tsc_unstable()) {
+		if (kvm_check_tsc_unstable() &&
+		    !vcpu->kvm->arch.tsc_immutable) {
 			u64 offset = kvm_compute_l1_tsc_offset(vcpu,
 						vcpu->arch.last_guest_tsc);
 			kvm_vcpu_write_tsc_offset(vcpu, offset);
@@ -4386,7 +4396,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		 * On a host with synchronized TSC, there is no need to update
 		 * kvmclock on vcpu->cpu migration
 		 */
-		if (!vcpu->kvm->arch.use_master_clock || vcpu->cpu == -1)
+		if ((!vcpu->kvm->arch.use_master_clock || vcpu->cpu == -1) &&
+		    !vcpu->kvm->arch.tsc_immutable)
 			kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
 		if (vcpu->cpu != cpu)
 			kvm_make_request(KVM_REQ_MIGRATE_TIMER, vcpu);
@@ -5352,10 +5363,11 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 		break;
 	}
 	case KVM_SET_TSC_KHZ: {
-		u32 user_tsc_khz;
+		u32 user_tsc_khz = (u32)arg;
 
 		r = -EINVAL;
-		user_tsc_khz = (u32)arg;
+		if (vcpu->kvm->arch.tsc_immutable)
+			goto out;
 
 		if (kvm_has_tsc_control &&
 		    user_tsc_khz >= kvm_max_guest_tsc_khz)
@@ -10994,9 +11006,12 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
 
 	if (mutex_lock_killable(&vcpu->mutex))
 		return;
-	vcpu_load(vcpu);
-	kvm_synchronize_tsc(vcpu, 0);
-	vcpu_put(vcpu);
+
+	if (!kvm->arch.tsc_immutable) {
+		vcpu_load(vcpu);
+		kvm_synchronize_tsc(vcpu, 0);
+		vcpu_put(vcpu);
+	}
 
 	/* poll control enabled by default */
 	vcpu->arch.msr_kvm_poll_control = 1;
@@ -11253,6 +11268,10 @@ int kvm_arch_hardware_enable(void)
 	if (backwards_tsc) {
 		u64 delta_cyc = max_tsc - local_tsc;
 		list_for_each_entry(kvm, &vm_list, vm_list) {
+			if (vcpu->kvm->arch.tsc_immutable) {
+				pr_warn_ratelimited("Backwards TSC observed and guest with immutable TSC active\n");
+				continue;
+			}
 			kvm->arch.backwards_tsc_observed = true;
 			kvm_for_each_vcpu(i, vcpu, kvm) {
 				vcpu->arch.tsc_offset_adjustment += delta_cyc;
-- 
2.25.1