linux-kernel - Re: [PATCH v2 3/4] KVM: LAPIC: Apply change to TDCR right away to the timer

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Message-ID: <20171004124343.GD20277@flask>
Date:   Wed, 4 Oct 2017 14:43:43 +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 3/4] KVM: LAPIC: Apply change to TDCR right away to
 the timer

2017-10-04 09:59+0800, Wanpeng Li:
> 2017-10-04 1:28 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> > 2017-09-28 18:04-0700, Wanpeng Li:
> >> From: Wanpeng Li <wanpeng.li@...mail.com>
> >>
> >> The description in the Intel SDM of how the divide configuration
> >> register is used: "The APIC timer frequency will be the processor's bus
> >> clock or core crystal clock frequency divided by the value specified in
> >> the divide configuration register."
> >>
> >> Observation of baremetal shown that when the TDCR is change, the TMCCT
> >> does not change or make a big jump in value, but the rate at which it
> >> count down change.
> >>
> >> The patch update the emulation to APIC timer to so that a change to the
> >> divide configuration would be reflected in the value of the counter and
> >> when the next interrupt is triggered.
> >>
> >> Cc: Paolo Bonzini <pbonzini@...hat.com>
> >> Cc: Radim Krčmář <rkrcmar@...hat.com>
> >> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> >> ---
> >
> > Why do we need to do more than just restart the timer?
> 
> Because the current timer (hv or sw) are still running. I think the
> goal of this commit is to runtime update the rate of the current timer
> which is running. Our restart_apic_timer() implementation just cancels
> the current timer when switch between preemption timer and hrtimer.

I see ... we do need to know both divisors in order to make it work,
thanks.