Re: [v5 1/3] KVM: setup empty irq routing when create vm

[Christian Borntraeger]

Am 17.04.26 um 10:19 schrieb Christian Borntraeger:
> Am 06.05.24 um 12:17 schrieb Yi Wang:
> > From: Yi Wang <foxywang@xxxxxxxxxxx>
> >
> > Add a new function to setup empty irq routing in kvm path, which
> > can be invoded in non-architecture-specific functions. The difference
> > compared to the kvm_setup_empty_irq_routing() is this function just
> > alloc the empty irq routing and does not need synchronize srcu, as
> > we will call it in kvm_create_vm().
> >
> > Using the new adding function, we can setup empty irq routing when
> > kvm_create_vm(), so that x86 and s390 no longer need to set
> > empty/dummy irq routing when creating an IRQCHIP 'cause it avoid
> > an synchronize_srcu.
> >
> > Signed-off-by: Yi Wang <foxywang@xxxxxxxxxxx>
>
> We have recently looked into cpu consumption for virtio.
> So interestingly enough, this increases cpu consumption for things like uperf
> ping pong on s390.
> Bisect points to this commit.
> I originally thought that this is a no-op for s390, but it is not.
>
> The reasons seems to be that nr_rt_entries is now 1 instead of 0 making every
> interrupt inject more expensive as we no longer drop out in
>
> int kvm_irq_map_gsi(struct kvm *kvm,
>                      struct kvm_kernel_irq_routing_entry *entries, int gsi)
> {
>          struct kvm_irq_routing_table *irq_rt;
>          struct kvm_kernel_irq_routing_entry *e;
>          int n = 0;
>
>          irq_rt = srcu_dereference_check(kvm->irq_routing, &kvm->irq_srcu,
>                                          lockdep_is_held(&kvm->irq_lock));
>          if (irq_rt && gsi < irq_rt->nr_rt_entries) {  <---------
Hmm, I guess I misread the code and the problem is likely not this.
Let me have another look.