linux-kernel - Re: [PATCH v3 01/15] KVM: x86: Add a framework for enabling KVM-governed x86 features

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Message-ID: <ZNzbJ9Y+8Uon327c@google.com>
Date:   Wed, 16 Aug 2023 07:20:23 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Guang Zeng <guang.zeng@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Yuan Yao <yuan.yao@...el.com>
Subject: Re: [PATCH v3 01/15] KVM: x86: Add a framework for enabling
 KVM-governed x86 features

On Wed, Aug 16, 2023, Kai Huang wrote:
> > diff --git a/arch/x86/kvm/governed_features.h b/arch/x86/kvm/governed_features.h
> > new file mode 100644
> > index 000000000000..40ce8e6608cd
> > --- /dev/null
> > +++ b/arch/x86/kvm/governed_features.h
> > @@ -0,0 +1,9 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#if !defined(KVM_GOVERNED_FEATURE) || defined(KVM_GOVERNED_X86_FEATURE)
> > +BUILD_BUG()
> > +#endif
> > +
> > +#define KVM_GOVERNED_X86_FEATURE(x) KVM_GOVERNED_FEATURE(X86_FEATURE_##x)
> > +
> > +#undef KVM_GOVERNED_X86_FEATURE
> > +#undef KVM_GOVERNED_FEATURE
> 
> Nit:
> 
> Do you want to move the very last
> 
> 	#undef KVM_GOVERNED_FEATURE
> 
> out of "governed_features.h", but to the place(s) where the macro is defined?
> 
> Yes there will be multiple:
> 
> 	#define KVM_GOVERNED_FEATURE(x)	...
> 	#include "governed_features.h"
> 	#undef KVM_GOVERNED_FEATURE
> 
> But this looks clearer to me.

I agree the symmetry looks better, but doing the #undef in governed_features.h
is much more robust.  E.g. having the #undef in the header makes it all but impossible
to have a bug where we forget to #undef KVM_GOVERNED_FEATURE.  Or worse, have two
bugs where we forget to #undef and then also forget to #define in a later include
and consume the stale #define.

And I also want to follow the pattern used by kvm-x86-ops.h.