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 <[email protected]>
> Reported-by: kernel test robot <[email protected]>
> Signed-off-by: Sean Christopherson <[email protected]>
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 <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
> ---
> 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
>
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 <[email protected]>
> > Reported-by: kernel test robot <[email protected]>
> > Signed-off-by: Sean Christopherson <[email protected]>
>
> 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.
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 <[email protected]>
> > > Reported-by: kernel test robot <[email protected]>
> > > Signed-off-by: Sean Christopherson <[email protected]>
> >
> > 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