Re: [PATCH][RFC] KVM: x86: Don't reset deadline to period when timer is in one shot mode

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 17, 2022, Li,Rongqing wrote:
> > +Jim, Peter, and Wanpeng
> > 
> > On Wed, Oct 12, 2022, Li RongQing wrote:
> > > In one-shot mode, the APIC timer stops counting when the timer reaches
> > > zero, so don't reset deadline to period for one shot mode
> > >
> > > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > > ---
> > >  arch/x86/kvm/lapic.c | 8 ++++++--
> > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index
> > > 9dda989..bf39027 100644
> > > --- a/arch/x86/kvm/lapic.c
> > > +++ b/arch/x86/kvm/lapic.c
> > > @@ -1840,8 +1840,12 @@ static bool set_target_expiration(struct kvm_lapic
> > *apic, u32 count_reg)
> > >  		if (unlikely(count_reg != APIC_TMICT)) {
> > >  			deadline = tmict_to_ns(apic,
> > >  				     kvm_lapic_get_reg(apic, count_reg));
> > > -			if (unlikely(deadline <= 0))
> > > -				deadline = apic->lapic_timer.period;
> > > +			if (unlikely(deadline <= 0)) {
> > > +				if (apic_lvtt_period(apic))
> > > +					deadline = apic->lapic_timer.period;
> > > +				else
> > > +					deadline = 0;
> > > +			}
> > 
> > This is not the standard "count has reached zero" path, it's the "vCPU is
> > migrated and the timer needs to be resumed on the destination" path.
> > Zeroing the deadline here will not squash the timer, IIUC it will cause the timer
> > to immediately fire.
> > 
> > That said, I think the patch is actually correct 
> 
> Should we set deadline to 0 when it is expired whether the timer is one shot
> mode or period mode ?

I'm not sure I follow the question.  Are you asking if this path should set the
deadline to '0' even if the timer is in periodic mode?