MIME-Version: 1.0
From: Dmitry Vyukov <dvyukov@google.com>
Date: Tue, 12 Sep 2017 16:42:56 +0200
Message-ID: <CACT4Y+boDPGU_02AhzUCCHfHdPaviMd5sTJTuV5RuYNor4wRpg@mail.gmail.com>
Subject: "KVM: x86: generalize guest_cpuid_has_ helpers" breaks clang
To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, KVM list <kvm@vger.kernel.org>
Cc: llvmlinux@lists.linuxfoundation.org,
        Alexander Potapenko <glider@google.com>,
        andreyknvl <andreyknvl@google.com>, Michael Davidson <md@google.com>,
        Greg Hackmann <ghackmann@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>
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Sender: linux-kernel-owner@vger.kernel.org
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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