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Date: Mon, 10 Jun 2024 17:25:18 -0700
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Subject: Re: [v5 2/3] KVM: x86: don't setup empty irq routing when KVM_CAP_SPLIT_IRQCHIP
From: Sean Christopherson <seanjc@google.com>
To: Yi Wang <up2wing@gmail.com>
Cc: pbonzini@redhat.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, 
	dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, 
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org, wanpengli@tencent.com, 
	foxywang@tencent.com, oliver.upton@linux.dev, maz@kernel.org, 
	anup@brainfault.org, atishp@atishpatra.org, borntraeger@linux.ibm.com, 
	frankja@linux.ibm.com, imbrenda@linux.ibm.com, weijiang.yang@intel.com
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On Mon, May 06, 2024, Yi Wang wrote:
> From: Yi Wang <foxywang@tencent.com>
>=20
> 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.
>=20
> 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.
>=20
> As we have set up empty irq routing when creating vm, so this is no
> need now.
>=20
> Signed-off-by: Yi Wang <foxywang@tencent.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(-)
>=20
> 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);
> =20
>  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)
> =20
>  static const struct kvm_irq_routing_entry empty_routing[] =3D {};

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

arch/x86/kvm/irq_comm.c:398:43: error: =E2=80=98empty_routing=E2=80=99 defi=
ned but not used [-Werror=3Dunused-const-variable=3D]
  398 | static const struct kvm_irq_routing_entry empty_routing[] =3D {};

I'll fix this up when applying, too.