Re: [PATCH 2/4] KVM: arm64: Reset last_steal on vCPU pid change

On Mon, Aug 17, 2026 1:42:20AM -0700, Marc Zyngier wrote:
> On Sun, 16 Aug 2026 06:33:03 +0100,
> Dongli Zhang <[email protected]> wrote:
>>
>> The previous commit resets x86 steal time accounting when the vCPU PID is
>> changed. Do the same for arm64.
> 
> Drop this statement, it really doesn't provide any information.

Sure.

> 
>>
>> KVM keeps a vCPU fd alive when userspace hot-unplugs a vCPU. If the fd is
>> later reused from a new vCPU thread, vcpu->arch.steal.last_steal still
>> reflects the old task's run_delay, while current->sched_info.run_delay
>> belongs to the new task.
>>
>> Reset vcpu->arch.steal.last_steal from kvm_arch_vcpu_run_pid_change()
>> unconditionally so the next stolen time update computes its delta against
>> the new task's run_delay.
>>
>> Assisted-by: Codex:GPT-5.5
>> Signed-off-by: Dongli Zhang <[email protected]>
>> ---
>>  arch/arm64/include/asm/kvm_host.h | 1 +
>>  arch/arm64/kvm/arm.c              | 2 ++
>>  arch/arm64/kvm/pvtime.c           | 5 +++++
>>  3 files changed, 8 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index bae2c4f92ef5..4607f956e787 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -1339,6 +1339,7 @@ static inline bool kvm_arch_pmi_in_guest(struct kvm_vcpu *vcpu)
>>  long kvm_hypercall_pv_features(struct kvm_vcpu *vcpu);
>>  gpa_t kvm_init_stolen_time(struct kvm_vcpu *vcpu);
>>  void kvm_update_stolen_time(struct kvm_vcpu *vcpu);
>> +void kvm_reset_stolen_time(struct kvm_vcpu *vcpu);
>>  
>>  bool kvm_arm_pvtime_supported(void);
>>  int kvm_arm_pvtime_set_attr(struct kvm_vcpu *vcpu,
>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>> index 9a6c72a18672..0f6e63eace21 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -929,6 +929,8 @@ int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
>>  	if (!kvm_arm_vcpu_is_finalized(vcpu))
>>  		return -EPERM;
>>  
>> +	kvm_reset_stolen_time(vcpu);
>> +
>>  	if (likely(vcpu_has_run_once(vcpu)))
>>  		return 0;
>>  
>> diff --git a/arch/arm64/kvm/pvtime.c b/arch/arm64/kvm/pvtime.c
>> index 4ceabaa4c30b..000bf49cc0fd 100644
>> --- a/arch/arm64/kvm/pvtime.c
>> +++ b/arch/arm64/kvm/pvtime.c
>> @@ -32,6 +32,11 @@ void kvm_update_stolen_time(struct kvm_vcpu *vcpu)
>>  	srcu_read_unlock(&kvm->srcu, idx);
>>  }
>>  
>> +void kvm_reset_stolen_time(struct kvm_vcpu *vcpu)
>> +{
>> +	vcpu->arch.steal.last_steal = current->sched_info.run_delay;
>> +}
>> +
>>  long kvm_hypercall_pv_features(struct kvm_vcpu *vcpu)
>>  {
>>  	u32 feature = smccc_get_arg1(vcpu);
> 
> Why isn't this common code? I really don't see the point in making
> this arch-specific code. I'd expect something like this (untested):
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index ace5801a592f..3fb77360af01 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -950,6 +950,8 @@ struct kvm_vcpu_arch {
>  	pid_t pid;
>  };
>  
> +#define kvm_arch_vcpu_last_steal(v)	(v)->arch.steal.last_steal
> +
>  /*
>   * Each 'flag' is composed of a comma-separated triplet:
>   *
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 45e784462ec6..117aeb49231a 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4459,6 +4459,9 @@ static long kvm_vcpu_ioctl(struct file *filp,
>  			if (r)
>  				break;
>  
> +			if (IS_ENABLED(CONFIG_HAVE_PV_STEAL_CLOCK_GEN))
> +				kvm_arch_vcpu_last_steal(vcpu) = current->sched_info.run_delay;
> +
>  			newpid = get_task_pid(current, PIDTYPE_PID);
>  			write_lock(&vcpu->pid_lock);
>  			vcpu->pid = newpid;
> 
> where each architecture that implements steal time provides an
> accessor, and the core code is in charge of the adjustment.
> 
> It also makes sure that we don't leave any architecture behind.
> 
Or how about making it something like below?

if (IS_ENABLED(CONFIG_HAVE_PV_STEAL_CLOCK_GEN))
	kvm_arch_vcpu_reset_last_steal(vcpu);

That would still move the policy to common KVM code, while leaving the exact
arch state to the architecture implementation.

For x86, the hook can reset vcpu->arch.st.last_steal for regular KVM steal time.
If we also decide to cover Xen runstate in this series, the same x86 hook can
additionally reset vcpu->arch.xen.last_steal.

Thank you very much!

Dongli Zhang