linux-kernel - "KVM: x86: generalize guest_cpuid_has

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Date:   Tue, 12 Sep 2017 16:42:56 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>
Cc:     llvmlinux@...ts.linuxfoundation.org,
        Alexander Potapenko <glider@...gle.com>,
        andreyknvl <andreyknvl@...gle.com>,
        Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: "KVM: x86: generalize guest_cpuid_has_ helpers" breaks clang

Hi Radim,

I've just noticed that your commit "KVM: x86: generalize
guest_cpuid_has_ helpers" breaks clang build on this assert:

static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned x86_feature)
{
    unsigned x86_leaf = x86_feature / 32;

    BUILD_BUG_ON(!__builtin_constant_p(x86_leaf));

In clang __builtin_constant_p is never true for function arguments,
it's true only for compile-time constants (what you can use as stack
array size, or C++ template argument). What would work is an
additional macro along the lines of:

#define x86_feature_cpuid(x) (BUILD_BUG_ON(!__builtin_constant_p(x),
__x86_feature_cpuid(x))

But again assuming that caller pass the constant directly.

Could you please fix it?

Thanks