Re: [PATCH 35/40] libxl: convert libxlMigrationDstArgs to GObject

On Mon, May 18, 2020 at 05:16:29PM +0200, Rafael Fonseca wrote: > On Mon, May 18, 2020 at 3:28 PM Rafael Fonseca <r4f4rfs(a)gmail.com&gt; wrote: > > > > On Mon, May 18, 2020 at 11:31 AM Daniel P. Berrangé <berrange(a)redhat.com&gt; wrote: > > > > > > On Sun, May 17, 2020 at 11:03:57PM +0200, Rafael Fonseca wrote: > > > > On Fri, May 15, 2020 at 5:24 PM Daniel P. Berrangé <berrange(a)redhat.com&gt; wrote: > > > > > Since we're using GitLab now, if you just fork the libvirt repo > > > > > and push a branch to your gitlab fork, it will run CI which will > > > > > check for these mistakes. > > > > > > > > Good to know. I did that and the CI is only failing now in the > > > > cross-compilation phase with 32-bit systems: > > > > > > > > ../../src/util/virstoragefile.h: In function 'VIR_STORAGE_SOURCE': > > > > /usr/i686-w64-mingw32/sys-root/mingw/include/glib-2.0/gobject/gtype.h:2301:6: > > > > error: cast increases required alignment of target type > > > > [-Werror=cast-align] > > > > 2301 | ((ct*) g_type_check_instance_cast ((GTypeInstance*) ip, gt)) > > > > | ^ > > > > /usr/i686-w64-mingw32/sys-root/mingw/include/glib-2.0/gobject/gtype.h:484:66: > > > > note: in expansion of macro '_G_TYPE_CIC' > > > > 484 | #define G_TYPE_CHECK_INSTANCE_CAST(instance, g_type, c_type) > > > > (_G_TYPE_CIC ((instance), (g_type), c_type)) > > > > | ^~~~~~~~~~~ > > > > /usr/i686-w64-mingw32/sys-root/mingw/include/glib-2.0/gobject/gtype.h:1412:12: > > > > note: in expansion of macro 'G_TYPE_CHECK_INSTANCE_CAST' > > > > 1412 | return G_TYPE_CHECK_INSTANCE_CAST (ptr, > > > > module_obj_name##_get_type (), ModuleObjName); } \ > > > > | ^~~~~~~~~~~~~~~~~~~~~~~~~~ > > > > ../../src/util/virstoragefile.h:390:1: note: in expansion of macro > > > > 'G_DECLARE_FINAL_TYPE' > > > > 390 | G_DECLARE_FINAL_TYPE(virStorageSource, vir_storage_source, VIR, > > > > STORAGE_SOURCE, GObject); > > > > | ^~~~~~~~~~~~~~~~~~~~ > > > > > > > > Any ideas on how to solve that? > > > > > > This is odd and I can't tell why. Assyuming only virStorageSource shows > > > the issue though, there must be something in the way the struct is > > > arranged that causes alignment issues. > > > > I disabled the virStorageSource patch and tried again. On armv7l it > > failed with the same error but in a different part: > > > > ../../src/qemu/qemu_conf.h: In function 'VIR_QEMU_DRIVER_CONFIG': > > Just to complete the information, I also disabled the QEmuDriverConfig > patch and the CI finished successfully in armv7l. So it's something in > virStorageSource and QemuDriverConfig, either the structs themselves > or something missing from my changes. You're not doing anything wrong. The compiler doesn't have enough info to realize that its warning is wrong. We have a "GObject *" which requires 4 byte alignment, and are trying to cast to "virStorageSource *" or "QemuDriverConfig *" which require 8 byte alignment. The compiler is pessimistic and has to consider that the original memory for the "GObject *" is only aligned to 4 byte boundary, and thus a bad cast would result if we cast to "virStorageSource *". What the compiler does not know is that any "GObject *" we have will always have been allocated using g_object_new, which delegates to g_slice_new, which uses malloc or a slab allocator. Both malloc & the glib slab allocator guarantee that all memory they allocate is aligned to 8 byte boundaries on 32-bit. So the only way the cast from GObject * -> virStorageSource * could be a bad cast is if we allocated the GObject on the stack but that'll never happen. So the cast alignment warnings are bogus. The difficulty is that I don't see a nice way to eliminate the warnings. Options I see include: 1 Turn off -Wcast-align entirely 2 Modify GLib to add attribute(align(8)) to GObject 3 Modify GLib to use Pragma push/pop to selectively disable -Wcast-align I think (2) is an ABI change is probably not possible. Possibly we have todo (1) and then also do (3) so we can revert (1) in future.