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

[Philippe Mathieu-Daudé]

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
>
> $ rpm -q gcc
> gcc-7.1.1-3.fc26.ppc64le
>
> Testing the size of args_tmp seems to be enough to prevent the warning
> to pop up.

This sizeof() use looks unnatural to me. I wonder why not use size_t, 
since this is about sizeof()/ARRAY_SIZE(). The problem seems to come 
from the commit this cast was introduced (61c7bbd236):

target-ppc/kvm.c:1302:21: error: comparison of unsigned expression < 0 
is always false [-Werror=type-limits]

So I'd rather suggest this code, which looks more natural to read to me:

    if (ARRAY_SIZE(args_tmp)) {
        for (i = 0; i < ARRAY_SIZE(args_tmp) && ...

I Cc'ed Eric :)

Regards,

Phil.

> Signed-off-by: Greg Kurz <address@hidden>
> ---
>   include/sysemu/kvm.h |   12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
> index 91fc07ee9afe..41fc1005a9ea 100644
> --- a/include/sysemu/kvm.h
> +++ b/include/sysemu/kvm.h
> @@ -429,9 +429,11 @@ int kvm_vm_check_extension(KVMState *s, unsigned int 
> extension);
>           };                                                           \
>           uint64_t args_tmp[] = { __VA_ARGS__ };                       \
>           int i;                                                       \
> -        for (i = 0; i < (int)ARRAY_SIZE(args_tmp) &&                 \
> +        if (sizeof(args_tmp)) {                                      \
> +            for (i = 0; i < (int)ARRAY_SIZE(args_tmp) &&             \
>                        i < ARRAY_SIZE(cap.args); i++) {                \
> -            cap.args[i] = args_tmp[i];                               \
> +                cap.args[i] = args_tmp[i];                           \
> +            }                                                        \
>           }                                                            \
>           kvm_vm_ioctl(s, KVM_ENABLE_CAP, &cap);                       \
>       })
> @@ -444,9 +446,11 @@ int kvm_vm_check_extension(KVMState *s, unsigned int 
> extension);
>           };                                                           \
>           uint64_t args_tmp[] = { __VA_ARGS__ };                       \
>           int i;                                                       \
> -        for (i = 0; i < (int)ARRAY_SIZE(args_tmp) &&                 \
> +        if (sizeof(args_tmp)) {                                      \
> +            for (i = 0; i < (int)ARRAY_SIZE(args_tmp) &&             \
>                        i < ARRAY_SIZE(cap.args); i++) {                \
> -            cap.args[i] = args_tmp[i];                               \
> +                cap.args[i] = args_tmp[i];                           \
> +            }                                                        \
>           }                                                            \
>           kvm_vcpu_ioctl(cpu, KVM_ENABLE_CAP, &cap);                   \
>       })