linux-kernel - Re: "KVM: x86: generalize guest_cpuid_has

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Date:   Tue, 12 Sep 2017 17:18:52 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        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: Re: "KVM: x86: generalize guest_cpuid_has_ helpers" breaks clang

2017-09-12 16:42+0200, Dmitry Vyukov:
> 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:

GCC optimizes it thanks to __always_inline, so the x86_feature is
constant in each instance of this function ... the goal is to have
compile-time input checking.

> #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.

The __builtin_constant_p() check is just a canary, the important ones
are

  BUILD_BUG_ON(x86_leaf >= ARRAY_SIZE(reverse_cpuid));
  BUILD_BUG_ON(reverse_cpuid[x86_leaf].function == 0);

and these would be very awkward if moved out of the function.

> Could you please fix it?

Sure, I can just make them BUG_ON (or WARN_ON with error handling), but
I tried with clang version 4.0.1 and got no errors -- are you using an
older version?  (or a command other than `make HOSTCC=clang CC=clang`)

Thanks.