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

From: Sean Christopherson <seanjc@google.com>
To: Nathan Chancellor <nathan@kernel.org>
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 17:59:05 +0000	[thread overview]
Message-ID: <YhZ16cMMcHQIvS9d@google.com> (raw)
In-Reply-To: <YhZuk8eA6rsDuJkd@dev-arch.archlinux-ax161>

On Wed, Feb 23, 2022, Nathan Chancellor wrote:
> 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?

IMO, no.  I think it's completely reasonable for the compiler to complain that KVM
is generating two different pointer types out of a ternary operator.

clang is somewhat inconsistent, though it may be deliberate.  clang doesn't complain
about implicitly casting a 'void *' to another data type, e.g. this complies clean,
where "data" is a 'void *'

	struct kvm_vcpu *x = vcpu ? : data;

But changing it to a function on the lhs triggers the warn:

	typeof(kvm_x86_ops.vcpu_run) x = kvm_x86_ops.vcpu_run ? : data;

Again, complaining in the function pointer case seems reasonable.