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

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Message-ID: <20171003172833.GC21107@flask>
Date:   Tue, 3 Oct 2017 19:28:34 +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 v2 3/4] KVM: LAPIC: Apply change to TDCR right away to
 the timer

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?

The TMCCT should remain roughly at the same level -- changing divide
count modifies target_expiration and it looks like apic_get_tmcct()
would get the same result like before changing divide count.

Thanks.