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

[Greg Kurz <groug@kaod.org>]

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.

Signed-off-by: Greg Kurz <groug@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);                   \
     })