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

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Date:   Wed, 09 Mar 2022 10:26:51 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH v7 1/2] KVM: LAPIC: Make lapic timer unpinned

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?

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