Re: [PATCH] KVM: x86: Fix pointer mistmatch warning when patching RET0 static calls

From: Nathan Chancellor <nathan@kernel.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, llvm@lists.linux.dev,
	linux-kernel@vger.kernel.org, Like Xu <like.xu.linux@gmail.com>
Subject: Re: [PATCH] KVM: x86: Fix pointer mistmatch warning when patching RET0 static calls
Date: Wed, 23 Feb 2022 10:27:47 -0700	[thread overview]
Message-ID: <YhZuk8eA6rsDuJkd@dev-arch.archlinux-ax161> (raw)
In-Reply-To: <20220223162355.3174907-1-seanjc@google.com>

Hi Sean,

On Wed, Feb 23, 2022 at 04:23:55PM +0000, Sean Christopherson wrote:
> Cast kvm_x86_ops.func to 'void *' when updating KVM static calls that are
> conditionally patched to __static_call_return0().  clang complains about
> using mismatching pointers in the ternary operator, which breaks the
> build when compiling with CONFIG_KVM_WERROR=y.
> 
>   >> arch/x86/include/asm/kvm-x86-ops.h:82:1: warning: pointer type mismatch
>   ('bool (*)(struct kvm_vcpu *)' and 'void *') [-Wpointer-type-mismatch]
> 
> Fixes: 5be2226f417d ("KVM: x86: allow defining return-0 static calls")
> Reported-by: Like Xu <like.xu.linux@gmail.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Thank you for the patch! Is this a bug in clang? I ended up creating a
small reproducer in case we need to file a bug with clang upstream:

https://godbolt.org/z/P7nEdzejE

This does shut up the warning and I can still spawn guests on my AMD and
Intel systems so:

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  arch/x86/include/asm/kvm_host.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 713e08f62385..f285ddb8b66b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1547,8 +1547,8 @@ static inline void kvm_ops_static_call_update(void)
>  	WARN_ON(!kvm_x86_ops.func); __KVM_X86_OP(func)
>  #define KVM_X86_OP_OPTIONAL __KVM_X86_OP
>  #define KVM_X86_OP_OPTIONAL_RET0(func) \
> -	static_call_update(kvm_x86_##func, kvm_x86_ops.func ? : \
> -			   (void *) __static_call_return0);
> +	static_call_update(kvm_x86_##func, (void *)kvm_x86_ops.func ? : \
> +					   (void *)__static_call_return0);
>  #include <asm/kvm-x86-ops.h>
>  #undef __KVM_X86_OP
>  }
> 
> base-commit: f4bc051fc91ab9f1d5225d94e52d369ef58bec58
> -- 
> 2.35.1.473.g83b2b277ed-goog
> 