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

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Date:   Wed, 4 Oct 2017 15:21:41 +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 1/4] KVM: LAPIC: Fix lapic timer mode transition

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
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.