[PATCH v4 38/47] x86/kvmclock: Refactor handling of PVCLOCK_TSC_STABLE_BIT during kvmclock_init()

[Sean Christopherson <seanjc@xxxxxxxxxx>]

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.

Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
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 6372b4dc7b0c..4e304f1c887d 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -305,7 +305,7 @@ static __init void kvm_sched_clock_init(bool stable)
 
 void __init kvmclock_init(bool prefer_tsc)
 {
-       u8 flags;
+       bool stable = false;
 
        if (!kvm_para_available() || !kvmclock)
                return;
@@ -332,11 +332,18 @@ void __init kvmclock_init(bool prefer_tsc)
        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);
 
        if (!x86_init.hyper.get_tsc_khz)
                x86_init.hyper.get_tsc_khz = kvmclock_get_tsc_khz;
-- 
2.54.0.823.g6e5bcc1fc9-goog