linux-kernel - Re: [PATCH v2 25/49] KVM: x86: Harden CPU capabilities processing against out-of-scope features

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Message-ID: <Zo19SWre5eJm8XTu@google.com>
Date: Tue, 9 Jul 2024 11:11:21 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Hou Wenlong <houwenlong.hwl@...group.com>, 
	Kechen Lu <kechenl@...dia.com>, Oliver Upton <oliver.upton@...ux.dev>, 
	Binbin Wu <binbin.wu@...ux.intel.com>, Yang Weijiang <weijiang.yang@...el.com>, 
	Robert Hoo <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v2 25/49] KVM: x86: Harden CPU capabilities processing
 against out-of-scope features

On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
> > +/*
> > + * For kernel-defined leafs, mask the boot CPU's pre-populated value.  For KVM-
> > + * defined leafs, explicitly set the leaf, as KVM is the one and only authority.
> > + */
> > +#define kvm_cpu_cap_init(leaf, mask)					\
> > +do {									\
> > +	const struct cpuid_reg cpuid = x86_feature_cpuid(leaf * 32);	\
> > +	const u32 __maybe_unused kvm_cpu_cap_init_in_progress = leaf;	\
> 
> Why not to #define the kvm_cpu_cap_init_in_progress as well instead of a variable?

Macros can't #define new macros.  A macro could be used, but it would require the
caller to #define and #undef the macro, e.g.

	#define kvm_cpu_cap_init_in_progress CPUID_1_ECX
	kvm_cpu_cap_init(CPUID_1_ECX, ...)
	#undef kvm_cpu_cap_init_in_progress

but, stating the obvious, that's ugly and is less robust than automatically
"defining" the in-progress leaf in kvm_cpu_cap_init().