linux-kernel - Re: [v5 2/3] KVM: x86: don't setup empty irq routing when KVM_CAP_SPLIT

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Date: Mon, 10 Jun 2024 17:25:18 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yi Wang <up2wing@...il.com>
Cc: pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, wanpengli@...cent.com, 
	foxywang@...cent.com, oliver.upton@...ux.dev, maz@...nel.org, 
	anup@...infault.org, atishp@...shpatra.org, borntraeger@...ux.ibm.com, 
	frankja@...ux.ibm.com, imbrenda@...ux.ibm.com, weijiang.yang@...el.com
Subject: Re: [v5 2/3] KVM: x86: don't setup empty irq routing when KVM_CAP_SPLIT_IRQCHIP

On Mon, May 06, 2024, Yi Wang wrote:
> From: Yi Wang <foxywang@...cent.com>
> 
> We found that it may cost more than 20 milliseconds very accidentally
> to enable cap of KVM_CAP_SPLIT_IRQCHIP on a host which has many vms
> already.
> 
> The reason is that when vmm(qemu/CloudHypervisor) invokes
> KVM_CAP_SPLIT_IRQCHIP kvm will call synchronize_srcu_expedited() and
> might_sleep and kworker of srcu may cost some delay during this period.
> 
> As we have set up empty irq routing when creating vm, so this is no
> need now.
> 
> Signed-off-by: Yi Wang <foxywang@...cent.com>
> ---
>  arch/x86/kvm/irq.h      | 1 -
>  arch/x86/kvm/irq_comm.c | 5 -----
>  arch/x86/kvm/x86.c      | 3 ---
>  3 files changed, 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/irq.h b/arch/x86/kvm/irq.h
> index c2d7cfe82d00..76d46b2f41dd 100644
> --- a/arch/x86/kvm/irq.h
> +++ b/arch/x86/kvm/irq.h
> @@ -106,7 +106,6 @@ void __kvm_migrate_timers(struct kvm_vcpu *vcpu);
>  int apic_has_pending_timer(struct kvm_vcpu *vcpu);
>  
>  int kvm_setup_default_irq_routing(struct kvm *kvm);
> -int kvm_setup_empty_irq_routing(struct kvm *kvm);
>  int kvm_irq_delivery_to_apic(struct kvm *kvm, struct kvm_lapic *src,
>  			     struct kvm_lapic_irq *irq,
>  			     struct dest_map *dest_map);
> diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c
> index 68f3f6c26046..6ee7ca39466e 100644
> --- a/arch/x86/kvm/irq_comm.c
> +++ b/arch/x86/kvm/irq_comm.c
> @@ -397,11 +397,6 @@ int kvm_setup_default_irq_routing(struct kvm *kvm)
>  
>  static const struct kvm_irq_routing_entry empty_routing[] = {};

empty_routing is unused after this patch and can+should be removed, e.g.

arch/x86/kvm/irq_comm.c:398:43: error: ‘empty_routing’ defined but not used [-Werror=unused-const-variable=]
  398 | static const struct kvm_irq_routing_entry empty_routing[] = {};

I'll fix this up when applying, too.