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

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 09, 2026, Lei Chen wrote:
> 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@smartx.com>
> Reported-by: Jaroslav Pulchart <jaroslav.pulchart@gooddata.com>
> Closes: https://lore.kernel.org/all/CAK8fFZ5gY8_Mw2A=iZVFNVKQNrXQzVsn-HTd+Me9K6ZfmdgA+Q@mail.gmail.com/
> ---
> 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@smartx.com/)
> ---
>  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);

What I was trying to call out in my review of v1, is that prior to commit
446fcce2a52b, the effectively ratelimiting applied to *all* instances of
KVM_REQ_GLOBAL_CLOCK_UPDATE.  Which meant that KVM's existing behavior is that
kvm_write_system_time() would be subject to the ratelimiting as well.

That said, I don't see any obvious problems with immediately honoring writes to
MSR_KVM_SYSTEM_TIME{,_NEW}, and it's probably a much better experience for the
guest.  So I'm a-ok with this approach, but we should call out that skipping the
synthetic MSR case is deliberate.  No need for a v3, I'll add a blurb when
applying.

> +		}
> +
>  		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);

IIUC, so long was KVM doesn't explicitly invoke ratelimit_state_exit(), setting
RATELIMIT_MSG_ON_RELEASE means we won't get dmesg spam?  To be clear, I'm 100%
in favor of suppressing dmesg output.

>  	kvm->arch.kvmclock_offset = -get_kvmclock_base_ns();
>  
>  	raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
> -- 
> 2.51.0
>