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

[Paolo Bonzini]

On 29/04/21 00:56, Sean Christopherson wrote:
> On Wed, Apr 28, 2021, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>
> Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
>
> > ---
> >   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

Queued, thanks.

Paolo