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Date:   Thu, 10 Aug 2023 15:20:13 -0700
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Subject: Re: [PATCH v8 02/14] KVM: Declare kvm_arch_flush_remote_tlbs() globally
From:   Sean Christopherson <seanjc@google.com>
To:     Raghavendra Rao Ananta <rananta@google.com>
Cc:     Shaoqin Huang <shahuang@redhat.com>, Gavin Shan <gshan@redhat.com>,
        Oliver Upton <oliver.upton@linux.dev>,
        Marc Zyngier <maz@kernel.org>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Zenghui Yu <yuzenghui@huawei.com>,
        Anup Patel <anup@brainfault.org>,
        Atish Patra <atishp@atishpatra.org>,
        Jing Zhang <jingzhangos@google.com>,
        Reiji Watanabe <reijiw@google.com>,
        Colton Lewis <coltonlewis@google.com>,
        David Matlack <dmatlack@google.com>,
        Fuad Tabba <tabba@google.com>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-mips@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
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On Thu, Aug 10, 2023, Raghavendra Rao Ananta wrote:
> On Thu, Aug 10, 2023 at 5:26=E2=80=AFAM Shaoqin Huang <shahuang@redhat.co=
m> wrote:
> > On 8/10/23 00:38, Raghavendra Rao Ananta wrote:
> > >>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > >>> index e3f968b38ae97..ade5d4500c2ce 100644
> > >>> --- a/include/linux/kvm_host.h
> > >>> +++ b/include/linux/kvm_host.h
> > >>> @@ -1484,6 +1484,8 @@ static inline int kvm_arch_flush_remote_tlbs(=
struct kvm *kvm)
> > >>>    {
> > >>>        return -ENOTSUPP;
> > >>>    }
> > >>> +#else
> > >>> +int kvm_arch_flush_remote_tlbs(struct kvm *kvm);
> > >>>    #endif
> > >>>
> > >>>    #ifdef __KVM_HAVE_ARCH_NONCOHERENT_DMA
> > >>
> > >> Is the declaration inconsistent to that in arch/x86/include/asm/kvm_=
host.h?
> > >> In order to keep them consistent, I guess we need move kvm_arch_flus=
h_remote_tlbs()
> > >> from x86's header file to arch/x86/kvm/mmu/mmu.c and 'inline' needs =
to be dropped.
> > >>
> > > Unsure of the original intentions, I didn't want to disturb any
> > > existing arrangements. If more people agree to this refactoring, I'm
> > > happy to move.
> >
> > This is amazing to me. This change can be compiled without any error
> > even if the declaration inconsistent between the kvm_host.h and x86's
> > header file.
> >
> > I'm curious which option make it possible?
> >
> After doing some experiments, I think it works because of the order in
> which the inline-definition and the declaration are laid out. If the
> 'inline' part of the function comes first and then the declaration, we
> don't see any error. However if the positions were reversed, we would
> see an error. (I'm not sure what the technical reason for this is).
>=20
> Just to be safe, I can move the definition to arch/x86/kvm/mmu/mmu.c
> as a non-inline function.

No need, asm/kvm_host.h _must_ be included before the declaration, otherwis=
e the
declaration wouldn't be made because __KVM_HAVE_ARCH_FLUSH_REMOTE_TLBS woul=
dn't
be defined.  I.e. we won't run into issues where the non-static declaration=
 comes
before the static inline definition.

C99 explicitly covers this case:

  6.2.2 Linkages of identifiers

  ...

  If the declaration of a file scope identifier for an object or a function=
 contains the storage-
  class specifier static, the identifier has internal linkage.

  For an identifier declared with the storage-class specifier extern in a s=
cope in which a
  prior declaration of that identifier is visible if the prior declaration =
specifies internal or
  external linkage, the linkage of the identifier at the later declaration =
is the same as the
  linkage specified at the prior declaration. If no prior declaration is vi=
sible, or if the prior
  declaration specifies no linkage, then the identifier has external linkag=
e.

In short, because the "static inline" declared internal linkage first, it w=
ins.