[PATCH v9 10/21] KVM: x86: Fall back to non-master-clock if clockread fails in get_kvmclock()

[Sean Christopherson <seanjc@google.com>]

From: David Woodhouse <dwmw@amazon.co.uk>

When computing kvmclock and it's currently in master-clock mode, fall back
to the non-master-clock path if the clock read fails, e.g. if the kernel's
clocksource is transitioning away from TSC but ka->use_master_clock hasn't
been updated yet.

The rdtsc() fallback was added (well, kept) in commit c68dc1b577ea ("KVM:
x86: Report host tsc and realtime values in KVM_GET_CLOCK") purely to avoid
uninitialized variables and compilation problems on 32-bit kernels (already
addressed).  In hindsight, keeping the rdtsc() was a hack and a mistake.

Link: https://lore.kernel.org/all/CAOQ_QsgVqS_PuJo8F10Gg5Xw+tKt+5gDx+kJf1j3CiPO4MAOqg@mail.gmail.com
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
[sean: isolate from refactoring changes, write changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 304e078cb899..33ba840d3267 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1674,14 +1674,13 @@ static bool __get_kvmclock_master_clock(struct kvm *kvm,
 	if (!tsc_hz)
 		return false;
 
-	if (kvm_get_walltime_and_clockread(&ts, &data->host_tsc)) {
-		data->realtime = ts.tv_nsec + NSEC_PER_SEC * ts.tv_sec;
-		data->flags |= KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC;
-	} else {
-		data->host_tsc = rdtsc();
-	}
+	if (!kvm_get_walltime_and_clockread(&ts, &data->host_tsc))
+		return false;
+
+	data->realtime = ts.tv_nsec + NSEC_PER_SEC * ts.tv_sec;
+	data->flags |= KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC |
+		       KVM_CLOCK_TSC_STABLE;
 
-	data->flags |= KVM_CLOCK_TSC_STABLE;
 	hv_clock.tsc_timestamp = ka->master_cycle_now;
 	hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
 	kvm_get_time_scale(NSEC_PER_SEC,  tsc_hz,
-- 
2.55.0.679.g6767b8d81c-goog