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Date:   Wed, 19 Jul 2023 07:15:09 -0700
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Subject: Re: [RFC PATCH v11 05/29] KVM: Convert KVM_ARCH_WANT_MMU_NOTIFIER to CONFIG_KVM_GENERIC_MMU_NOTIFIER
From:   Sean Christopherson <seanjc@google.com>
To:     Yuan Yao <yuan.yao@linux.intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Paul Moore <paul@paul-moore.com>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-security-module@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Chao Peng <chao.p.peng@linux.intel.com>,
        Fuad Tabba <tabba@google.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Vishal Annapurve <vannapurve@google.com>,
        Ackerley Tng <ackerleytng@google.com>,
        Maciej Szmigiero <mail@maciej.szmigiero.name>,
        Vlastimil Babka <vbabka@suse.cz>,
        David Hildenbrand <david@redhat.com>,
        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>,
        Wang <wei.w.wang@intel.com>,
        Liam Merwick <liam.merwick@oracle.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
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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.