[PATCH v2 23/38] x86/kvmclock: Refactor handling of PVCLOCK_TSC_STABLE_BIT during kvmclock_init()

[Sean Christopherson]

Clean up the setting of PVCLOCK_TSC_STABLE_BIT during kvmclock init to
make it somewhat obvious that pvclock_read_flags() must be called *after*
pvclock_set_flags().

Note, in theory, a different PV clock could have set PVCLOCK_TSC_STABLE_BIT
in the supported flags, i.e. reading flags only if
KVM_FEATURE_CLOCKSOURCE_STABLE_BIT is set could very, very theoretically
result in a change in behavior.  In practice, the kernel only supports a
single PV clock.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kernel/kvmclock.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 934ee4a4c6d4..0580fe1aefa0 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -304,7 +304,7 @@ static void __init kvm_sched_clock_init(bool stable)
 
 void __init kvmclock_init(void)
 {
-	u8 flags;
+	bool stable = false;
 
 	if (!kvm_para_available() || !kvmclock)
 		return;
@@ -331,11 +331,18 @@ void __init kvmclock_init(void)
 	kvm_register_clock("primary cpu clock");
 	pvclock_set_pvti_cpu0_va(hv_clock_boot);
 
-	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))
+	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT)) {
 		pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);
 
-	flags = pvclock_read_flags(&hv_clock_boot[0].pvti);
-	kvm_sched_clock_init(flags & PVCLOCK_TSC_STABLE_BIT);
+		/*
+		 * Check if the clock is stable *after* marking TSC_STABLE as a
+		 * valid flag.
+		 */
+		stable = pvclock_read_flags(&hv_clock_boot[0].pvti) &
+			 PVCLOCK_TSC_STABLE_BIT;
+	}
+
+	kvm_sched_clock_init(stable);
 
 	tsc_register_calibration_routines(kvm_get_tsc_khz, kvm_get_tsc_khz);
 
-- 
2.48.1.711.g2feabab25a-goog