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

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Message-ID: <CANRm+CwoFANC8yZ=+GNv2TRc_veC6UyVVb23d9o8RDCKirvWXQ@mail.gmail.com>
Date:   Wed, 4 Oct 2017 21:50:22 +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 21:21 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?

> 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