KVM: x86: Use get_kvmclock() in kvm_get_wall_clock_epoch()

Now that get_kvmclock() correctly handles TSC scaling and captures both
wallclock and kvmclock from the same TSC reading,
kvm_get_wall_clock_epoch() can simply call it instead of duplicating
the pvclock computation.

This eliminates the last instance of the "definition C" kvmclock
calculation — as described in commit 633d7652f80f ("KVM: x86/xen: Do not
corrupt KVM clock in kvm_xen_shared_info_init()") — which computed
nanoseconds directly from the host TSC without accounting for guest TSC
scaling.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://patch.msgid.link/20260826213303.914988-13-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 9 insertions, 50 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4567896b7cf7..e0e13235a69e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1941,63 +1941,22 @@ int kvm_guest_time_update(struct kvm_vcpu *v)
  * wallclock and kvmclock times, and subtracting one from the other.
  *
  * Fall back to using their values at slightly different moments by
- * calling ktime_get_real_ns() and get_kvmclock_ns() separately.
+ * calling ktime_get_real_ns() and get_kvmclock() separately.
  */
 uint64_t kvm_get_wall_clock_epoch(struct kvm *kvm)
 {
-#ifdef CONFIG_X86_64
-	struct pvclock_vcpu_time_info hv_clock;
-	struct kvm_arch *ka = &kvm->arch;
-	unsigned long seq, local_tsc_khz;
-	struct timespec64 ts;
-	uint64_t host_tsc;
-
-	do {
-		seq = read_seqcount_begin(&ka->pvclock_sc);
-
-		local_tsc_khz = 0;
-		if (!ka->use_master_clock)
-			break;
-
-		/*
-		 * The TSC read and the call to get_cpu_tsc_khz() must happen
-		 * on the same CPU.
-		 */
-		get_cpu();
-
-		local_tsc_khz = get_cpu_tsc_khz();
-
-		if (local_tsc_khz &&
-		    !kvm_get_walltime_and_clockread(&ts, &host_tsc))
-			local_tsc_khz = 0; /* Fall back to old method */
-
-		put_cpu();
-
-		/*
-		 * These values must be snapshotted within the seqcount loop.
-		 * After that, it's just mathematics which can happen on any
-		 * CPU at any time.
-		 */
-		hv_clock.tsc_timestamp = ka->master_cycle_now;
-		hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
+	struct kvm_clock_data data;
 
-	} while (read_seqcount_retry(&ka->pvclock_sc, seq));
+	get_kvmclock(kvm, &data);
 
 	/*
-	 * If the conditions were right, and obtaining the wallclock+TSC was
-	 * successful, calculate the KVM clock at the corresponding time and
-	 * subtract one from the other to get the guest's epoch in nanoseconds
-	 * since 1970-01-01.
+	 * If get_kvmclock() captured both wallclock and kvmclock from the
+	 * same TSC reading, use them for a precise epoch calculation.
 	 */
-	if (local_tsc_khz) {
-		kvm_get_time_scale(NSEC_PER_SEC, local_tsc_khz * NSEC_PER_USEC,
-				   &hv_clock.tsc_shift,
-				   &hv_clock.tsc_to_system_mul);
-		return ts.tv_nsec + NSEC_PER_SEC * ts.tv_sec -
-			__pvclock_read_cycles(&hv_clock, host_tsc);
-	}
-#endif
-	return ktime_get_real_ns() - get_kvmclock_ns(kvm);
+	if (data.flags & KVM_CLOCK_REALTIME)
+		return data.realtime - data.clock;
+
+	return ktime_get_real_ns() - data.clock;
 }
 
 /*