linux-kernel - Re: [PATCH 2/2] KVM: LAPIC: micro-optimize fixed mode ipi delivery

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Date:   Mon, 11 Nov 2019 22:59:08 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 2/2] KVM: LAPIC: micro-optimize fixed mode ipi delivery

On 09/11/19 08:05, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> After disabling mwait/halt/pause vmexits, RESCHEDULE_VECTOR and
> CALL_FUNCTION_SINGLE_VECTOR etc IPI is one of the main remaining
> cause of vmexits observed in product environment which can't be
> optimized by PV IPIs. This patch is the follow-up on commit
> 0e6d242eccdb (KVM: LAPIC: Micro optimize IPI latency), to optimize
> redundancy logic before fixed mode ipi is delivered in the fast
> path.
> 
> - broadcast handling needs to go slow path, so the delivery mode repair
>   can be delayed to before slow path.

I agree with this part, but is the cost of the irq->shorthand check
really measurable?

Paolo

> - self-IPI will not be intervened by hypervisor any more after APICv is
>   introduced and the boxes support APICv are popular now. In addition,
>   kvm_apic_map_get_dest_lapic() can handle the self-IPI, so there is no
>   need a shortcut for the non-APICv case.
> 
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/irq_comm.c | 6 +++---
>  arch/x86/kvm/lapic.c    | 5 -----
>  2 files changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c
> index 8ecd48d..aa88156 100644
> --- a/arch/x86/kvm/irq_comm.c
> +++ b/arch/x86/kvm/irq_comm.c
> @@ -52,15 +52,15 @@ int kvm_irq_delivery_to_apic(struct kvm *kvm, struct kvm_lapic *src,
>  	unsigned long dest_vcpu_bitmap[BITS_TO_LONGS(KVM_MAX_VCPUS)];
>  	unsigned int dest_vcpus = 0;
>  
> +	if (kvm_irq_delivery_to_apic_fast(kvm, src, irq, &r, dest_map))
> +		return r;
> +
>  	if (irq->dest_mode == 0 && irq->dest_id == 0xff &&
>  			kvm_lowest_prio_delivery(irq)) {
>  		printk(KERN_INFO "kvm: apic: phys broadcast and lowest prio\n");
>  		irq->delivery_mode = APIC_DM_FIXED;
>  	}
>  
> -	if (kvm_irq_delivery_to_apic_fast(kvm, src, irq, &r, dest_map))
> -		return r;
> -
>  	memset(dest_vcpu_bitmap, 0, sizeof(dest_vcpu_bitmap));
>  
>  	kvm_for_each_vcpu(i, vcpu, kvm) {
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index b29d00b..ea936fa 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -951,11 +951,6 @@ bool kvm_irq_delivery_to_apic_fast(struct kvm *kvm, struct kvm_lapic *src,
>  
>  	*r = -1;
>  
> -	if (irq->shorthand == APIC_DEST_SELF) {
> -		*r = kvm_apic_set_irq(src->vcpu, irq, dest_map);
> -		return true;
> -	}
> -
>  	rcu_read_lock();
>  	map = rcu_dereference(kvm->arch.apic_map);
>  
>