Subject: Re: [RFC PATCH v11 05/29] KVM: Convert KVM_ARCH_WANT_MMU_NOTIFIER to CONFIG_KVM_GENERIC_MMU_NOTIFIER

From: Sean Christopherson <[email-masked]>
Date: Wed, 19 Jul 2023 07:15:09 -0700

On Wed, Jul 19, 2023, Yuan Yao wrote:
> On Tue, Jul 18, 2023 at 04:44:48PM -0700, Sean Christopherson wrote:
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 90a0be261a5c..d2d3e083ec7f 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -255,7 +255,9 @@ bool kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> >  int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu);
> >  #endif
> >
> > -#ifdef KVM_ARCH_WANT_MMU_NOTIFIER
> > +struct kvm_gfn_range;
> 
> Not sure why a declaration here, it's defined for ARCHs which defined
> KVM_ARCH_WANT_MMU_NOTIFIER before.

The forward declaration exists to handle cases where CONFIG_KVM=n, specifically
arch/powerpc/include/asm/kvm_ppc.h's declaration of hooks to forward calls to
uarch modules:

	bool (*unmap_gfn_range)(struct kvm *kvm, struct kvm_gfn_range *range);
	bool (*age_gfn)(struct kvm *kvm, struct kvm_gfn_range *range);
	bool (*test_age_gfn)(struct kvm *kvm, struct kvm_gfn_range *range);
	bool (*set_spte_gfn)(struct kvm *kvm, struct kvm_gfn_range *range);

Prior to using a Kconfig, a forward declaration wasn't necessary because
arch/powerpc/include/asm/kvm_host.h would #define KVM_ARCH_WANT_MMU_NOTIFIER even
if CONFIG_KVM=n.

Alternatively, kvm_ppc.h could declare the struct.  I went this route mainly to
avoid the possibility of someone encountering the same problem on a different
architecture.

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