Re: [PATCH] KVM: LAPIC: Accurately guarantee busy wait for timer to expire when using hv_timer

[Sean Christopherson <seanjc@google.com>]

On Wed, Apr 28, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> Commit ee66e453db13d (KVM: lapic: Busy wait for timer to expire when 
> using hv_timer) tries to set ktime->expired_tscdeadline by checking 
> ktime->hv_timer_in_use since lapic timer oneshot/periodic modes which 
> are emulated by vmx preemption timer also get advanced, they leverage 
> the same vmx preemption timer logic with tsc-deadline mode. However, 
> ktime->hv_timer_in_use is cleared before apic_timer_expired() handling, 
> let's delay this clearing in preemption-disabled region.
> 
> Fixes: ee66e453db13d (KVM: lapic: Busy wait for timer to expire when using hv_timer)

Well that's embarassing.  I suspect/hope I tested the case where start_hv_timer()
detects the timer already expired.  On the plus side, start_hv_timer() calls
cancel_hv_timer() after apic_timer_expired(), so there are unlikely to be hidden
side effects (and cancel_hv_timer() is tiny).

> Cc: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>

Reviewed-by: Sean Christopherson <seanjc@google.com>

> ---
>  arch/x86/kvm/lapic.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 152591f..c0ebef5 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1913,8 +1913,8 @@ void kvm_lapic_expired_hv_timer(struct kvm_vcpu *vcpu)
>  	if (!apic->lapic_timer.hv_timer_in_use)
>  		goto out;
>  	WARN_ON(rcuwait_active(&vcpu->wait));
> -	cancel_hv_timer(apic);
>  	apic_timer_expired(apic, false);
> +	cancel_hv_timer(apic);
>  
>  	if (apic_lvtt_period(apic) && apic->lapic_timer.period) {
>  		advance_periodic_target_expiration(apic);
> -- 
> 2.7.4
>