linux-kernel - Re: [PATCH v7 1/2] KVM: LAPIC: Make lapic timer unpinned

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Date:   Wed, 9 Mar 2022 08:24:21 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v7 1/2] KVM: LAPIC: Make lapic timer unpinned

On Wed, Mar 09, 2022 at 10:26:51AM +0100, David Woodhouse wrote:
> On Sat, 2019-07-06 at 09:26 +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.com>
> > 
> > Commit 61abdbe0bcc2 ("kvm: x86: make lapic hrtimer pinned") pinned the
> > lapic timer to avoid to wait until the next kvm exit for the guest to
> > see KVM_REQ_PENDING_TIMER set. There is another solution to give a kick
> > after setting the KVM_REQ_PENDING_TIMER bit, make lapic timer unpinned
> > will be used in follow up patches.
> > 
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Radim Krčmář <rkrcmar@...hat.com>
> > Cc: Marcelo Tosatti <mtosatti@...hat.com>
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> >  arch/x86/kvm/lapic.c | 8 ++++----
> >  arch/x86/kvm/x86.c   | 6 +-----
> >  2 files changed, 5 insertions(+), 9 deletions(-)
> 
> ...
> 
> 
> > @@ -2510,7 +2510,7 @@ void __kvm_migrate_apic_timer(struct kvm_vcpu *vcpu)
> >  
> >  	timer = &vcpu->arch.apic->lapic_timer.timer;
> >  	if (hrtimer_cancel(timer))
> > -		hrtimer_start_expires(timer, HRTIMER_MODE_ABS_PINNED);
> > +		hrtimer_start_expires(timer, HRTIMER_MODE_ABS);
> >  }
> >  
> >  /*
> 
> Wait, in that case why are we even bothering to cancel and restart the
> timer? I thought the whole point of that was to pin it to the *new* CPU
> that this vCPU is running on.
> 
> If not, can't we just kill __kvm_migrate_apic_timer() off completely
> not?

Current code looks like this:

void __kvm_migrate_apic_timer(struct kvm_vcpu *vcpu)
{
        struct hrtimer *timer;

        if (!lapic_in_kernel(vcpu) ||
                kvm_can_post_timer_interrupt(vcpu)) <----------
                return;

        timer = &vcpu->arch.apic->lapic_timer.timer;
        if (hrtimer_cancel(timer))
                hrtimer_start_expires(timer, HRTIMER_MODE_ABS_HARD);
}

Yeah, should be HRTIMER_MODE_ABS_PINNED AFAICS.