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 11:13:23 -0700	[thread overview]
Message-ID: <YhZ5Q8DNoGGWUBLh@dev-arch.archlinux-ax161> (raw)
In-Reply-To: <YhZ16cMMcHQIvS9d@google.com>

On Wed, Feb 23, 2022 at 05:59:05PM +0000, Sean Christopherson wrote:
> 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;

Right, I would assume this is deliberate. I think warning in this case
might be quite noisy, as the kernel implicitly converts 'void *' to
typed pointers for certain function pointer callbacks (although this
particular case is probably pretty rare).

> 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.

Ack, thank you for the clarification and explanation!

Cheers,
Nathan