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.
Paolo
> Signed-off-by: Greg Kurz <gr...@kaod.org>
> ---
> 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); \
> })
>
>
