linux-kernel - Re: [PATCH] KVM: LAPIC: Reset timer_advance

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Date:   Mon, 31 Aug 2020 14:48:12 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: LAPIC: Reset timer_advance_ns if timer mode switch

Wanpeng Li <kernellwp@...il.com> writes:

> From: Wanpeng Li <wanpengli@...cent.com>
>
> per-vCPU timer_advance_ns should be set to 0 if timer mode is not tscdeadline 
> otherwise we waste cpu cycles in the function lapic_timer_int_injected(), 

lapic_timer_int_injected is just a test, kvm_wait_lapic_expire()
(__kvm_wait_lapic_expire()) maybe?

> especially on AMD platform which doesn't support tscdeadline mode. We can 
> reset timer_advance_ns to the initial value if switch back to
> tscdealine

'tscdeadline'

> timer mode.
>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/lapic.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 654649b..abc296d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1499,10 +1499,16 @@ 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;
> +			if (timer_mode == APIC_LVT_TIMER_TSCDEADLINE &&
> +				lapic_timer_advance_dynamic)
> +				apic->lapic_timer.timer_advance_ns = LAPIC_TIMER_ADVANCE_NS_INIT;
>  		}
>  		apic->lapic_timer.timer_mode = timer_mode;
>  		limit_periodic_timer_frequency(apic);
>  	}
> +	if (timer_mode != APIC_LVT_TIMER_TSCDEADLINE &&
> +		lapic_timer_advance_dynamic)
> +		apic->lapic_timer.timer_advance_ns = 0;
>  }
>  
>  /*

-- 
Vitaly