linux-kernel - Re: [PATCH v6 2/3] KVM: LAPIC: Keep timer running when switching between one-shot and periodic mode

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Date:   Fri, 6 Oct 2017 17:04:34 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v6 2/3] KVM: LAPIC: Keep timer running when switching
 between one-shot and periodic mode

2017-10-06 15:17+0200, Radim Krčmář:
> 2017-10-05 18:54-0700, Wanpeng Li:
> > From: Wanpeng Li <wanpeng.li@...mail.com>
> > 
> > If we take TSC-deadline mode timer out of the picture, the Intel SDM
> > does not say that the timer is disable when the timer mode is change,
> > either from one-shot to periodic or vice versa.
> > 
> > After this patch, the timer is no longer disarmed on change of mode, so
> > the counter (TMCCT) keeps counting down.
> > 
> > So what does a write to LVTT changes ? On baremetal, the change of mode
> > is probably taken into account only when the counter reach 0. When this
> > happen, LVTT is use to figure out if the counter should restard counting
> > down from TMICT (so periodic mode) or stop counting (if one-shot mode).
> > 
> > This patch is based on observation of the behavior of the APIC timer on
> > baremetal as well as check that they does not go against the description
> > written in the Intel SDM.
> > 
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Radim Krčmář <rkrcmar@...hat.com>
> > Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> > ---
> >  arch/x86/kvm/lapic.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> Queued the first two patches, thanks.
> 
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 8841bb5..14f63b3 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -1329,10 +1329,14 @@ static void apic_update_lvtt(struct kvm_lapic *apic)
> >  
> >  	if (apic->lapic_timer.timer_mode != timer_mode) {
> >  		if (apic_lvtt_tscdeadline(apic) != (timer_mode ==
> > -				APIC_LVT_TIMER_TSCDEADLINE))
> > +				APIC_LVT_TIMER_TSCDEADLINE)) {
> >  			kvm_lapic_set_reg(apic, APIC_TMICT, 0);
> > +			hrtimer_cancel(&apic->lapic_timer.timer);
> > +		}
> > +		if (apic_lvtt_oneshot(apic) && (timer_mode ==
> > +				APIC_LVT_TIMER_PERIODIC))
> > +			limit_periodic_timer_frequency(apic);

I noticed a problem here that required slight changes (the current code
never actually did the rate limiting), please see kvm/queue.

> >  		apic->lapic_timer.timer_mode = timer_mode;
> > -		hrtimer_cancel(&apic->lapic_timer.timer);
> >  	}
> >  }
> >  
> > -- 
> > 2.7.4
> >