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 15:17:40 +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

2017-10-05 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);
>  		apic->lapic_timer.timer_mode = timer_mode;
> -		hrtimer_cancel(&apic->lapic_timer.timer);
>  	}
>  }
>  
> -- 
> 2.7.4
>