linux-kernel - Re: [PATCH v3 10/12] KVM: x86/mmu: Use BUILD_BUG_ON_INVALID() for KVM_MMU_WARN

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Message-ID: <ZMmYHQwWMpT8s9Vi@google.com>
Date:   Tue, 1 Aug 2023 16:41:17 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Philippe Mathieu-Daudé" <philmd@...aro.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mingwei Zhang <mizhang@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v3 10/12] KVM: x86/mmu: Use BUILD_BUG_ON_INVALID() for
 KVM_MMU_WARN_ON() stub

On Tue, Aug 01, 2023, Philippe Mathieu-Daudé wrote:
> Hi Sean,
> 
> On 29/7/23 02:47, Sean Christopherson wrote:
> > Use BUILD_BUG_ON_INVALID() instead of an empty do-while loop to stub out
> > KVM_MMU_WARN_ON() when CONFIG_KVM_PROVE_MMU=n, that way _some_ build
> > issues with the usage of KVM_MMU_WARN_ON() will be dected even if the
> > kernel is using the stubs, e.g. basic syntax errors will be detected.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >   arch/x86/kvm/mmu/mmu_internal.h | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> > index 40e74db6a7d5..f1ef670058e5 100644
> > --- a/arch/x86/kvm/mmu/mmu_internal.h
> > +++ b/arch/x86/kvm/mmu/mmu_internal.h
> > @@ -9,7 +9,7 @@
> >   #ifdef CONFIG_KVM_PROVE_MMU
> >   #define KVM_MMU_WARN_ON(x) WARN_ON_ONCE(x)
> >   #else
> > -#define KVM_MMU_WARN_ON(x) do { } while (0)
> > +#define KVM_MMU_WARN_ON(x) BUILD_BUG_ON_INVALID(x)
> 
> No need to include <linux/build_bug.h> ?

It's indirectly included via

  linux/kvm_host.h => linux/bug.h => linux/build_bug.h

Depending on the day, I might argue for explicitly including all dependencies, but
in this case build_bug.h is a "core" header, and IMO there's no value added by
including it directly.