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

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Message-ID: <0e8f1e1a9653d95332cc8f0d461c3015ae20d03e.camel@infradead.org>
Date:   Wed, 09 Mar 2022 16:03:54 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v7 1/2] KVM: LAPIC: Make lapic timer unpinned

On Wed, 2022-03-09 at 08:24 -0300, Marcelo Tosatti wrote:
> 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.

No, it's *intentionally* not pinned any more, since this patch that I'm
replying to, which became commit 4d151bf3b89. It doesn't *have* to run
on the same physical CPU, because of the epiphany that it can just call
kvm_vcpu_kick() after making the request.

But if it's a recurring timer it's still *best* for it to run on the
same physical CPU, just for cache locality reasons. So I think I was
wrong; the migration *isn't* pointless. It's still a valid
optimisation; it's just not *mandatory* any more.



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