linux-kernel - Re: [PATCH v8 02/14] KVM: Declare kvm_arch_flush_remote

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Date:   Thu, 10 Aug 2023 15:20:13 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     Shaoqin Huang <shahuang@...hat.com>, Gavin Shan <gshan@...hat.com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>,
        Jing Zhang <jingzhangos@...gle.com>,
        Reiji Watanabe <reijiw@...gle.com>,
        Colton Lewis <coltonlewis@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Fuad Tabba <tabba@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-mips@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v8 02/14] KVM: Declare kvm_arch_flush_remote_tlbs() globally

On Thu, Aug 10, 2023, Raghavendra Rao Ananta wrote:
> On Thu, Aug 10, 2023 at 5:26 AM Shaoqin Huang <shahuang@...hat.com> 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_flush_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).
> 
> 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, otherwise the
declaration wouldn't be made because __KVM_HAVE_ARCH_FLUSH_REMOTE_TLBS wouldn'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 scope 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 visible, or if the prior
  declaration specifies no linkage, then the identifier has external linkage.

In short, because the "static inline" declared internal linkage first, it wins.