Re: [Qemu-devel] [PATCH] kvm: workaround build break on gcc-7.1.1 / fedora26

[Paolo Bonzini <pbonzini@redhat.com>]

On 03/08/2017 16:24, Cornelia Huck wrote:
> On Thu, 3 Aug 2017 09:10:29 -0500
> Eric Blake <eblake@redhat.com> wrote:
> 
>> On 08/03/2017 08:46 AM, Philippe Mathieu-Daudé wrote:
>>> Hi Greg,
>>>
>>> On 08/02/2017 11:47 AM, Greg Kurz wrote:  
>>>> Building QEMU on fedora26 with the latest gcc package fails:
>>>>
>>>>    CC      ppc64-softmmu/target/ppc/kvm.o
>>>> In file included from include/sysemu/hw_accel.h:16:0,
>>>>                   from target/ppc/kvm.c:31:
>>>> target/ppc/kvm.c: In function ‘kvmppc_booke_watchdog_enable’:
>>>> include/sysemu/kvm.h:449:35: error: ‘args_tmp[i]’ may be used
>>>> uninitialized
>>>>   in this function [-Werror=maybe-uninitialized]
>>>>               cap.args[i] = args_tmp[i];                               \
>>>>                                     ^
>>>> target/ppc/kvm.c: In function ‘kvmppc_set_papr’:
>>>> include/sysemu/kvm.h:449:35: error: ‘args_tmp[i]’ may be used
>>>> uninitialized
>>>>   in this function [-Werror=maybe-uninitialized]
>>>> cc1: all warnings being treated as errors  
>>>
>>> I'm trying to reproduce this in our docker images (all x86_64 based) but
>>> can't reproduce.  
>>
>> That's because x86_64 hosts only call kvm_vm_enable_cap() with non-empty
>> varargs.  
> 
> There's
> 
> target/i386/kvm.c:            kvm_vcpu_enable_cap(cs, KVM_CAP_HYPERV_SYNIC, 0)) {
> 
>> But we have:
>>
>> accel/kvm/kvm-all.c: ret = kvm_vm_enable_cap(s, KVM_CAP_S390_IRQCHIP, 0);
>>
>> which is only compiled on s390 hosts (or, at least that's my guess,
>> based on the cap name)
> 
> I don't see how the compiler can optimize this away, as the check for
> this cap is an ioctl...

Indeed.  This is a compiler bug and it only provides a warning (meaning
--disable-werror silences it).  I don't think it's a good idea to uglify
the code for something that the compiler should easily optimize away...

Paolo

> This seems a bit ugly. And I still don't understand why this only seems
> to hit on ppc...