Re: [Qemu-devel] [PATCH v2] kvm: workaround build break on gcc-7.1.1 / fedora26

[Philippe Mathieu-Daudé]

On Mon, Aug 7, 2017 at 8:55 AM, Paolo Bonzini <address@hidden> wrote:
> On 07/08/2017 13:36, 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
>>
>> $ rpm -q gcc
>> gcc-7.1.1-3.fc26.ppc64le
>>
>> The compiler should obviously optimize this code away when no extra
>> agument is passed to kvm_vm_enable_cap() and kvm_vcpu_enable_cap(),
>> but it doesn't. This bug should be fixed one day in gcc, but we can
>> also change our code pattern so that we don't hit the issue anymore.
>> We workaround this, by using memcpy() instead of open-coding the copy.
>
> Nice way to do it, thanks.  I'll queue it for 2.10.

Indeed :)

>
> Paolo
>
>> Signed-off-by: Greg Kurz <address@hidden>

Reviewed-by: Philippe Mathieu-Daudé <address@hidden>

>> ---
>> v2: - use memcpy()
>> ---
>>  include/sysemu/kvm.h |   14 ++++----------
>>  1 file changed, 4 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
>> index 91fc07ee9afe..3a458f50e9f4 100644
>> --- a/include/sysemu/kvm.h
>> +++ b/include/sysemu/kvm.h
>> @@ -428,11 +428,8 @@ int kvm_vm_check_extension(KVMState *s, unsigned int 
>> extension);
>>              .flags = cap_flags,                                      \
>>          };                                                           \
>>          uint64_t args_tmp[] = { __VA_ARGS__ };                       \
>> -        int i;                                                       \
>> -        for (i = 0; i < (int)ARRAY_SIZE(args_tmp) &&                 \
>> -                     i < ARRAY_SIZE(cap.args); i++) {                \
>> -            cap.args[i] = args_tmp[i];                               \
>> -        }                                                            \
>> +        size_t n = MIN(ARRAY_SIZE(args_tmp), ARRAY_SIZE(cap.args));  \
>> +        memcpy(cap.args, args_tmp, n * sizeof(cap.args[0]));         \
>>          kvm_vm_ioctl(s, KVM_ENABLE_CAP, &cap);                       \
>>      })
>>
>> @@ -443,11 +440,8 @@ int kvm_vm_check_extension(KVMState *s, unsigned int 
>> extension);
>>              .flags = cap_flags,                                      \
>>          };                                                           \
>>          uint64_t args_tmp[] = { __VA_ARGS__ };                       \
>> -        int i;                                                       \
>> -        for (i = 0; i < (int)ARRAY_SIZE(args_tmp) &&                 \
>> -                     i < ARRAY_SIZE(cap.args); i++) {                \
>> -            cap.args[i] = args_tmp[i];                               \
>> -        }                                                            \
>> +        size_t n = MIN(ARRAY_SIZE(args_tmp), ARRAY_SIZE(cap.args));  \
>> +        memcpy(cap.args, args_tmp, n * sizeof(cap.args[0]));         \
>>          kvm_vcpu_ioctl(cpu, KVM_ENABLE_CAP, &cap);                   \
>>      })
>>
>>
>