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. 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) - and THAT code is passing empty varargs, which
explains args_tmp[] being a 0-length array, and getting the compiler to
complain about i < 0 always being false.
So my question on IRC was whether we can stack the decks, and force a
non-empty args_tmp = { 0, __VA_ARGS__} coupled by skipping the first
iteration in the for loop. Or, since cap.args[] is already being
zero-initialized, args_tmp = { __VA_ARGS__, 0 } means the last iteration
of the for loop is a no-op (assigning 0 to something that is already 0)
- although that may be harder to correctly account for both empty and
non-empty __VA_ARGS__.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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