[PATCH v2] KVM: x86: Rate-limit global clock updates on vCPU load

[Lei Chen]

commit 446fcce2a52b ("Revert "x86: kvm: rate-limit global clock updates"")
dropped the rate limiting for KVM_REQ_GLOBAL_CLOCK_UPDATE.

As a result, kvm_arch_vcpu_load() can queue global clock update requests
every time a vCPU is scheduled when the master clock is disabled or when
the vCPU is loaded for the first time.

Restore the throttling with a per-VM ratelimit state and gate
KVM_REQ_GLOBAL_CLOCK_UPDATE through __ratelimit(), so frequent vCPU
scheduling does not generate a steady stream of redundant clock update
requests.

Fixes: 446fcce2a52b ("Revert "x86: kvm: rate-limit global clock updates"")
Signed-off-by: Lei Chen <lei.chen@xxxxxxxxxx>
Reported-by: Jaroslav Pulchart <jaroslav.pulchart@xxxxxxxxxxxx>
Closes: https://lore.kernel.org/all/CAK8fFZ5gY8_Mw2A=iZVFNVKQNrXQzVsn-HTd+Me9K6ZfmdgA+Q@xxxxxxxxxxxxxx/
---
CHANGELOG:
v2:
- remove comment of kvmclock_update_rs
- make sure kvm_arch_vcpu_load make KVM_REQ_CLOCK_UPDATE for this vcpu
- add RATELIMIT_MSG_ON_RELEASE to kvmclock_update_rs

v1:
- initial version(https://lore.kernel.org/all/20260407070046.2336-1-lei.chen@xxxxxxxxxx/)
---
 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/x86.c              | 11 +++++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 5a3bfa293e8b..5e750c49d21e 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1453,6 +1453,7 @@ struct kvm_arch {
 	bool use_master_clock;
 	u64 master_kernel_ns;
 	u64 master_cycle_now;
+	struct ratelimit_state kvmclock_update_rs;
 
 #ifdef CONFIG_KVM_HYPERV
 	struct kvm_hv hyperv;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 63afdb6bb078..a534e8391611 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5210,8 +5210,13 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		 * On a host with synchronized TSC, there is no need to update
 		 * kvmclock on vcpu->cpu migration
 		 */
-		if (!vcpu->kvm->arch.use_master_clock || vcpu->cpu == -1)
-			kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
+		if (!vcpu->kvm->arch.use_master_clock || vcpu->cpu == -1) {
+			if (__ratelimit(&vcpu->kvm->arch.kvmclock_update_rs))
+				kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
+			else
+				kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
+		}
+
 		if (vcpu->cpu != cpu)
 			kvm_make_request(KVM_REQ_MIGRATE_TIMER, vcpu);
 		vcpu->cpu = cpu;
@@ -13189,6 +13194,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	raw_spin_lock_init(&kvm->arch.tsc_write_lock);
 	mutex_init(&kvm->arch.apic_map_lock);
 	seqcount_raw_spinlock_init(&kvm->arch.pvclock_sc, &kvm->arch.tsc_write_lock);
+	ratelimit_state_init(&kvm->arch.kvmclock_update_rs, HZ, 10);
+	ratelimit_set_flags(&kvm->arch.kvmclock_update_rs, RATELIMIT_MSG_ON_RELEASE);
 	kvm->arch.kvmclock_offset = -get_kvmclock_base_ns();
 
 	raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
-- 
2.51.0