linux-kernel - Re: [PATCH v2 2/4] KVM: LAPIC: Keep timer running when switching between one-shot and periodic mode

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Date:   Wed, 4 Oct 2017 15:33:41 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.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 v2 2/4] KVM: LAPIC: Keep timer running when switching
 between one-shot and periodic mode

2017-10-04 09:46+0800, Wanpeng Li:
> 2017-10-04 1:06 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> > 2017-09-28 18:04-0700, 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.
> >
> > I think it does, please see comment under [v2 1/4].
> 
> As I replied to [v2 1/4].

Right, so we probably shouldn't disable the timer.

> >> 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 | 40 ++++++++++++++++++++++++++++------------
> >>  1 file changed, 28 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> >> index a739cbb..946c11b 100644
> >> --- a/arch/x86/kvm/lapic.c
> >> +++ b/arch/x86/kvm/lapic.c
> >> @@ -1301,7 +1301,7 @@ static void update_divide_count(struct kvm_lapic *apic)
> >>                                  apic->divide_count);
> >>  }
> >>
> >> -static void apic_update_lvtt(struct kvm_lapic *apic)
> >> +static bool apic_update_lvtt(struct kvm_lapic *apic)
> >>  {
> >>       u32 timer_mode = kvm_lapic_get_reg(apic, APIC_LVTT) &
> >>                       apic->lapic_timer.timer_mode_mask;
> >> @@ -1309,7 +1309,9 @@ static void apic_update_lvtt(struct kvm_lapic *apic)
> >>       if (apic->lapic_timer.timer_mode != timer_mode) {
> >>               apic->lapic_timer.timer_mode = timer_mode;
> >>               hrtimer_cancel(&apic->lapic_timer.timer);
> >> +             return true;
> >>       }
> >> +     return false;
> >>  }
> >>
> >>  static void apic_timer_expired(struct kvm_lapic *apic)
> >> @@ -1729,7 +1744,8 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
> >>                       val |= APIC_LVT_MASKED;
> >>               val &= (apic_lvt_mask[0] | apic->lapic_timer.timer_mode_mask);
> >>               kvm_lapic_set_reg(apic, APIC_LVTT, val);
> >> -             apic_update_lvtt(apic);
> >> +             if (apic_update_lvtt(apic) && !apic_lvtt_tscdeadline(apic))
> >> +                     start_apic_timer(apic, true);

Changing the timer from one-shot to periodic doesn't change the expected
expiration -- I think we could instead skip hrtimer_cancel() in
apic_update_lvtt().