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:   Sat, 7 Oct 2017 08:50:55 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <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 23:04 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 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.

I see, thanks for the change.

Regards,
Wanpeng Li

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