Re: [PATCH v2] KVM: LAPIC: Fix cancel preemption timer repeatedly due to preemption

[Paolo Bonzini]

On 24/07/2017 10:57, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> 
> Preemption can occur in the preemption timer expiration handler:
> 
>           CPU0                    CPU1
> 
>   preemption timer vmexit
>   handle_preemption_timer(vCPU0)
>     kvm_lapic_expired_hv_timer
>       hv_timer_is_use == true
>   sched_out
>                            sched_in
>                            kvm_arch_vcpu_load
>                              kvm_lapic_restart_hv_timer
>                                restart_apic_timer
>                                  start_hv_timer
>                                    already-expired timer or sw timer triggerd in the window
>                                  start_sw_timer
>                                    cancel_hv_timer

At this point, the timer interrupt is injected, right?

If this is correct, kvm_lapic_expired_hv_timer can just do nothing if
the timer is not in use, with a comment explaining that the preemption
notifier has run start_sw_timer and thus injected the timer interrupt.

>                            /* back in kvm_lapic_expired_hv_timer */
>                            cancel_hv_timer
>                              WARN_ON(!apic->lapic_timer.hv_timer_in_use);  ==> Oops
> 
> This can be reproduced if CONFIG_PREEMPT is enabled.
> 
> This patch fixes it by don't cancel preemption timer repeatedly if 
> the preemption timer has already been cancelled due to preemption 
> since already-expired timer or sw timer triggered in the window.
> 
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
>  arch/x86/kvm/lapic.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 2819d4c..8341b40 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1560,9 +1560,13 @@ void kvm_lapic_expired_hv_timer(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_lapic *apic = vcpu->arch.apic;
>  
> -	WARN_ON(!apic->lapic_timer.hv_timer_in_use);
> -	WARN_ON(swait_active(&vcpu->wq));
> -	cancel_hv_timer(apic);
> +	preempt_disable();
> +	if (!(!apic_lvtt_period(apic) && atomic_read(&apic->lapic_timer.pending))) {

Why is the "if" necessary?

Maybe all of kvm_lapic_expired_hv_timer and start_sw_timer should be in
preemption-disabled regions, which trivially avoids any reentrancy issue
with the preempt notifier.  Then, cancel_hv_timer can assert that it's
called with preemption disabled.

Paolo

> +		WARN_ON(!apic->lapic_timer.hv_timer_in_use);
> +		WARN_ON(swait_active(&vcpu->wq));
> +		cancel_hv_timer(apic);
> +	}
> +	preempt_enable();
>  	apic_timer_expired(apic);
>  
>  	if (apic_lvtt_period(apic) && apic->lapic_timer.period) {
>