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.
