KVM: x86: Refactor tsc synchronization code

Refactor kvm_synchronize_tsc to make a new function that allows callers
to specify TSC parameters (offset, value, nanoseconds, etc.) explicitly
for the sake of participating in TSC synchronization.
Signed-off-by: Oliver Upton <oupton@google.com>
Message-Id: <20210916181538.968978-7-oupton@google.com>
[Make sure kvm->arch.cur_tsc_generation and vcpu->arch.this_tsc_generation are
 equal at the end of __kvm_synchronize_tsc, if matched is false. Reported by
 Maxim Levitsky. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -2435,13 +2435,63 @@ static inline bool kvm_check_tsc_unstable(void)
 	return check_tsc_unstable();
 }
 
+/*
+ * Infers attempts to synchronize the guest's tsc from host writes. Sets the
+ * offset for the vcpu and tracks the TSC matching generation that the vcpu
+ * participates in.
+ */
+static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
+				  u64 ns, bool matched)
+{
+	struct kvm *kvm = vcpu->kvm;
+
+	lockdep_assert_held(&kvm->arch.tsc_write_lock);
+
+	/*
+	 * We also track th most recent recorded KHZ, write and time to
+	 * allow the matching interval to be extended at each write.
+	 */
+	kvm->arch.last_tsc_nsec = ns;
+	kvm->arch.last_tsc_write = tsc;
+	kvm->arch.last_tsc_khz = vcpu->arch.virtual_tsc_khz;
+
+	vcpu->arch.last_guest_tsc = tsc;
+
+	kvm_vcpu_write_tsc_offset(vcpu, offset);
+
+	if (!matched) {
+		/*
+		 * We split periods of matched TSC writes into generations.
+		 * For each generation, we track the original measured
+		 * nanosecond time, offset, and write, so if TSCs are in
+		 * sync, we can match exact offset, and if not, we can match
+		 * exact software computation in compute_guest_tsc()
+		 *
+		 * These values are tracked in kvm->arch.cur_xxx variables.
+		 */
+		kvm->arch.cur_tsc_generation++;
+		kvm->arch.cur_tsc_nsec = ns;
+		kvm->arch.cur_tsc_write = tsc;
+		kvm->arch.cur_tsc_offset = offset;
+		kvm->arch.nr_vcpus_matched_tsc = 0;
+	} else if (vcpu->arch.this_tsc_generation != kvm->arch.cur_tsc_generation) {
+		kvm->arch.nr_vcpus_matched_tsc++;
+	}
+
+	/* Keep track of which generation this VCPU has synchronized to */
+	vcpu->arch.this_tsc_generation = kvm->arch.cur_tsc_generation;
+	vcpu->arch.this_tsc_nsec = kvm->arch.cur_tsc_nsec;
+	vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
+
+	kvm_track_tsc_matching(vcpu);
+}
+
 static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
 {
 	struct kvm *kvm = vcpu->kvm;
 	u64 offset, ns, elapsed;
 	unsigned long flags;
-	bool matched;
-	bool already_matched;
+	bool matched = false;
 	bool synchronizing = false;
 
 	raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
@@ -2487,48 +2537,9 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
 			offset = kvm_compute_l1_tsc_offset(vcpu, data);
 		}
 		matched = true;
-		already_matched = (vcpu->arch.this_tsc_generation == kvm->arch.cur_tsc_generation);
-	} else {
-		/*
-		 * We split periods of matched TSC writes into generations.
-		 * For each generation, we track the original measured
-		 * nanosecond time, offset, and write, so if TSCs are in
-		 * sync, we can match exact offset, and if not, we can match
-		 * exact software computation in compute_guest_tsc()
-		 *
-		 * These values are tracked in kvm->arch.cur_xxx variables.
-		 */
-		kvm->arch.cur_tsc_generation++;
-		kvm->arch.cur_tsc_nsec = ns;
-		kvm->arch.cur_tsc_write = data;
-		kvm->arch.cur_tsc_offset = offset;
-		matched = false;
-	}
-
-	/*
-	 * We also track th most recent recorded KHZ, write and time to
-	 * allow the matching interval to be extended at each write.
-	 */
-	kvm->arch.last_tsc_nsec = ns;
-	kvm->arch.last_tsc_write = data;
-	kvm->arch.last_tsc_khz = vcpu->arch.virtual_tsc_khz;
-
-	vcpu->arch.last_guest_tsc = data;
-
-	/* Keep track of which generation this VCPU has synchronized to */
-	vcpu->arch.this_tsc_generation = kvm->arch.cur_tsc_generation;
-	vcpu->arch.this_tsc_nsec = kvm->arch.cur_tsc_nsec;
-	vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
-
-	kvm_vcpu_write_tsc_offset(vcpu, offset);
-
-	if (!matched) {
-		kvm->arch.nr_vcpus_matched_tsc = 0;
-	} else if (!already_matched) {
-		kvm->arch.nr_vcpus_matched_tsc++;
 	}
 
-	kvm_track_tsc_matching(vcpu);
+	__kvm_synchronize_tsc(vcpu, offset, data, ns, matched);
 	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
 }