[RFC PATCH 6/8] KVM: x86: Replace pvclock_gtod_data vclock_mode with boolean

[David Woodhouse]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

The remaining users of pvclock_gtod_data only need to know whether
the host clocksource is TSC-based. Replace all vclock_mode checks
with a simple kvm_host_has_tsc_clocksource boolean, updated by the
pvclock_gtod_notify callback.

This is inherently racy (as it always was — kvm_track_tsc_matching
never held the gtod seqcount), relying on eventual consistency: the
notifier fires on every timekeeping update and will correct any
transient inconsistency within one tick.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Assisted-by: Kiro:claude-opus-4.6-1m
---
 arch/x86/kvm/x86.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d057f42603e4..c31b19860c13 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2649,6 +2649,8 @@ static s64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns)
 }
 
 #ifdef CONFIG_X86_64
+static bool kvm_host_has_tsc_clocksource;
+
 static inline bool gtod_is_based_on_tsc(int mode)
 {
 	return mode == VDSO_CLOCKMODE_TSC || mode == VDSO_CLOCKMODE_HVCLOCK;
@@ -2682,7 +2684,6 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation)
 {
 #ifdef CONFIG_X86_64
 	struct kvm_arch *ka = &vcpu->kvm->arch;
-	struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
 
 	/*
 	 * Track whether all vCPUs have matching TSC offsets (for
@@ -2701,7 +2702,7 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation)
 	 * accounts for its offset.
 	 */
 	bool use_master_clock = kvm_use_master_clock(vcpu->kvm) &&
-				gtod_is_based_on_tsc(gtod->clock.vclock_mode);
+				kvm_host_has_tsc_clocksource;
 
 	/*
 	 * Request a masterclock update if the masterclock needs to be toggled
@@ -2715,7 +2716,7 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation)
 
 	trace_kvm_track_tsc(vcpu->vcpu_id, ka->nr_vcpus_matched_tsc,
 			    atomic_read(&vcpu->kvm->online_vcpus),
-		            ka->use_master_clock, gtod->clock.vclock_mode);
+		            ka->use_master_clock, kvm_host_has_tsc_clocksource);
 #endif
 }
 
@@ -2836,7 +2837,7 @@ static inline bool kvm_check_tsc_unstable(void)
 	 * TSC is marked unstable when we're running on Hyper-V,
 	 * 'TSC page' clocksource is good.
 	 */
-	if (pvclock_gtod_data.clock.vclock_mode == VDSO_CLOCKMODE_HVCLOCK)
+	if (kvm_host_has_tsc_clocksource)
 		return false;
 #endif
 	return check_tsc_unstable();
@@ -3292,7 +3293,7 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
 					   &ka->master_tsc_mul);
 	}
 
-	vclock_mode = pvclock_gtod_data.clock.vclock_mode;
+	vclock_mode = kvm_host_has_tsc_clocksource;
 	trace_kvm_update_master_clock(ka->use_master_clock, vclock_mode,
 					ka->all_vcpus_matched_freq);
 #endif
@@ -10364,12 +10365,15 @@ static int pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
 	update_pvclock_gtod(tk);
 
 #ifdef CONFIG_X86_64
+	kvm_host_has_tsc_clocksource =
+		gtod_is_based_on_tsc(tk->tkr_mono.clock->vdso_clock_mode);
+
 	/*
 	 * Disable master clock if host does not trust, or does not use,
 	 * TSC based clocksource. Delegate queue_work() to irq_work as
 	 * this is invoked with tk_core.seq write held.
 	 */
-	if (!gtod_is_based_on_tsc(pvclock_gtod_data.clock.vclock_mode) &&
+	if (!kvm_host_has_tsc_clocksource &&
 	    atomic_read(&kvm_guest_has_master_clock) != 0)
 		irq_work_queue(&pvclock_irq_work);
 #endif
-- 
2.54.0