linux-kernel - Re: [PATCH v2 1/4] KVM: LAPIC: Fix lapic timer mode transition

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Message-ID: <CANRm+Cy0mRmwBXmvagL2-D8DpYxF3JQgUT1mnP0VQWZx4DZaCA@mail.gmail.com>
Date:   Wed, 4 Oct 2017 22:02:17 +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 v2 1/4] KVM: LAPIC: Fix lapic timer mode transition

2017-10-04 21:50 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> 2017-10-04 21:21 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>> 2017-10-04 09:45+0800, Wanpeng Li:
>>> 2017-10-04 1:05 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>>> > 2017-09-28 18:04-0700, Wanpeng Li:
>>> >> From: Wanpeng Li <wanpeng.li@...mail.com>
>>> >>
>>> >> SDM 10.5.4.1 TSC-Deadline Mode mentioned that "Transitioning between TSC-Deadline
>>> >> mode and other timer modes also disarms the timer". So the APIC Timer Initial Count
>>> >> Register for one-shot/periodic mode should be reset. This patch do it.
>>> >
>>> > At the beginning of the secion is also:
>>> >
>>> >   A write to the LVT Timer Register that changes the timer mode disarms
>>> >   the local APIC timer. The supported timer modes are given in Table
>>> >   10-2. The three modes of the local APIC timer are mutually exclusive.
>>>
>>> Yeah, I saw it before sending out the patches, but it is mentioned in
>>> TSC-deadline section which looks strange, if the timer is still
>>> disarmed when switching between one-shot and periodic mode before
>>> TSC-deadline is introduced and w/o TSC-deadline section？
>>
>> Yeah, maybe it is only true if the machine has TSC.  APM doesn't mention
>
> If APM is another emulator?

The document from AMD.

>
>> disarming at all.  Bochs only disables the timer it on switch from/to
>> TSC-deadline.
>>
>>> > So we should also disarm when switching between one-shot and periodic.
>>> >
>>> > apic_update_lvtt() already has logic to determine whether the timer mode
>>> > has changed and is the perfect place to clear APIC_TMICT.
>>>
>>> Agreed, thanks for your review, Radim. :)
>>
>> Bochs doesn't write 0 to APIC_TMICT, but it seems that Xen guys verified
>> that on bare-metal, so the behavior is fine.
>> Please just move it to apic_update_lvtt(),
>>
>> thanks.
>
> Ok, thanks for the review. :)
>
> Regards,
> Wanpeng Li