linux-kernel - Re: [PATCH] KVM: LAPIC: Disarm LAPIC timer includes pending timer around TSC deadline switch

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Date:   Tue, 10 May 2022 13:43:41 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: LAPIC: Disarm LAPIC timer includes pending timer
 around TSC deadline switch

On Tue, May 10, 2022, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> The timer is disarmed when switching between TSC deadline and other modes, 
> however, the pending timer is still in-flight, so let's accurately set 
> everything to a disarmed state.
> 
> Fixes: 4427593258 (KVM: x86: thoroughly disarm LAPIC timer around TSC deadline switch)
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/lapic.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 66b0eb0bda94..0274d17d91c2 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1562,6 +1562,7 @@ static void apic_update_lvtt(struct kvm_lapic *apic)
>  			kvm_lapic_set_reg(apic, APIC_TMICT, 0);
>  			apic->lapic_timer.period = 0;
>  			apic->lapic_timer.tscdeadline = 0;
> +			atomic_set(&apic->lapic_timer.pending, 0);

What about doing this in cancel_apic_timer()?  That seems to be a more natural
place to clear pending.  It's somewhat redundant since the other two callers of
cancel_apic_timer() start the timer immediately after, i.e. clear pending anyways,
but IMO it's odd to leave pending set when canceling the timer.

>  		}
>  		apic->lapic_timer.timer_mode = timer_mode;
>  		limit_periodic_timer_frequency(apic);
> -- 
> 2.25.1
>