linux-kernel - Re: [PATCH MANUALSEL 5.4 2/2] KVM: LAPIC: Enable timer posted-interrupt only when mwait/hlt is advertised

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Message-ID: <acab87c6-cbfc-63e1-1bc0-7f963432c412@redhat.com>
Date:   Wed, 27 Apr 2022 18:18:54 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Aili Yao <yaoaili@...gsoft.com>,
        Sean Christopherson <seanjc@...gle.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.4 2/2] KVM: LAPIC: Enable timer
 posted-interrupt only when mwait/hlt is advertised

On 4/27/22 17:54, Sasha Levin wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> [ Upstream commit 1714a4eb6fb0cb79f182873cd011a8ed60ac65e8 ]
> 
> As commit 0c5f81dad46 ("KVM: LAPIC: Inject timer interrupt via posted
> interrupt") mentioned that the host admin should well tune the guest
> setup, so that vCPUs are placed on isolated pCPUs, and with several pCPUs
> surplus for *busy* housekeeping.  In this setup, it is preferrable to
> disable mwait/hlt/pause vmexits to keep the vCPUs in non-root mode.
> 
> However, if only some guests isolated and others not, they would not
> have any benefit from posted timer interrupts, and at the same time lose
> VMX preemption timer fast paths because kvm_can_post_timer_interrupt()
> returns true and therefore forces kvm_can_use_hv_timer() to false.
> 
> By guaranteeing that posted-interrupt timer is only used if MWAIT or
> HLT are done without vmexit, KVM can make a better choice and use the
> VMX preemption timer and the corresponding fast paths.
> 
> Reported-by: Aili Yao <yaoaili@...gsoft.com>
> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
> Cc: Aili Yao <yaoaili@...gsoft.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> Message-Id: <1643112538-36743-1-git-send-email-wanpengli@...cent.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   arch/x86/kvm/lapic.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index afe3b8e61514..3696b4de9d99 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -118,7 +118,8 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
>   
>   bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu)
>   {
> -	return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> +	return pi_inject_timer && kvm_vcpu_apicv_active(vcpu) &&
> +		(kvm_mwait_in_guest(vcpu->kvm) || kvm_hlt_in_guest(vcpu->kvm));
>   }
>   EXPORT_SYMBOL_GPL(kvm_can_post_timer_interrupt);
>   

Acked-by: Paolo Bonzini <pbonzini@...hat.com>