From: Yi Wang <foxywang@tencent.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@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(-)

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[] = {};
 
-int kvm_setup_empty_irq_routing(struct kvm *kvm)
-{
-	return kvm_set_irq_routing(kvm, empty_routing, 0, 0);
-}
-
 void kvm_arch_post_irq_routing_update(struct kvm *kvm)
 {
 	if (!irqchip_split(kvm))
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 91478b769af0..01270182757b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6527,9 +6527,6 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 			goto split_irqchip_unlock;
 		if (kvm->created_vcpus)
 			goto split_irqchip_unlock;
-		r = kvm_setup_empty_irq_routing(kvm);
-		if (r)
-			goto split_irqchip_unlock;
 		/* Pairs with irqchip_in_kernel. */
 		smp_wmb();
 		kvm->arch.irqchip_mode = KVM_IRQCHIP_SPLIT;
-- 
2.39.3

On Mon, May 06, 2024, Yi Wang wrote:
> From: Yi Wang <foxywang@tencent.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@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(-)
> 
> 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.

On Tue, Jun 11, 2024 at 8:25 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Mon, May 06, 2024, Yi Wang wrote:
> > From: Yi Wang <foxywang@tencent.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@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(-)
> >
> > 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.


Many thanks for your work, Sean.


---
Best wishes
Yi Wang