Greg Kurz <gr...@kaod.org> writes: > Despite its simple name and common usage of "getting a pointer to > the machine" in system-mode emulation, qdev_get_machine() has some > subtilities. > > First, it can be called when running user-mode emulation : this is > because user-mode partly relies on qdev to instantiate its CPU > model. > > Second, but not least, it has a side-effect : if it cannot find an > object at "/machine" in the QOM tree, it creates a dummy "container" > object and put it there. A simple check on the type returned by > qdev_get_machine() allows user-mode to run the common qdev code, > skipping the parts that only make sense for system-mode. > > This side-effect turns out to complicate the use of qdev_get_machine() > for the system-mode case though. Most notably, qdev_get_machine() must > not be called before the machine object is added to the QOM tree by > qemu_create_machine(), otherwise the existing dummy "container" object > would cause qemu_create_machine() to fail with something like :
Stupid trap. > Unexpected error in object_property_try_add() at ../../qom/object.c:1223: > qemu-system-ppc64: attempt to add duplicate property 'machine' to > object (type 'container') > Aborted (core dumped) > > This situation doesn't exist in the current code base, mostly because > of preventive fixing of some "latent bugs" in QEMU 4.0 (see 1a3ec8c1564 > and e2fb3fbbf9c for details). I lacked the stamina to address the root problem: automatic creation of dummy containers where real ones may be needed. Is /machine the only such container? Have you reviewed the other uses of container_get()? > A new kind of breakage was spotted very recently though : > > $ ./qemu-system-ppc64 -device power8_v2.0-spapr-cpu-core,help > /home/thuth/devel/qemu/include/hw/boards.h:24: > MACHINE: Object 0x5635bd53af10 is not an instance of type machine > Aborted (core dumped) > > This comes from the change 3df261b6676b in QEMU 5.0. It unwillingly > added a new condition for qdev_get_machine() to be called too early, > breaking MACHINE(qdev_get_machine()) in generic cpu-core code this > time. > > In order to avoid further subtle breakages like this, change the > implentation of qdev_get_machine() to: > - keep the existing behaviour of creating the dummy "container" > object for the user-mode case only ; > - abort() if the machine doesn't exist yet in the QOM tree for > the system-mode case. This gives a precise hint to developpers > that calling qdev_get_machine() too early is a programming bug. In other words, we fail right away instead of planting a landmine for later. Good. The alternative would be mandating "must create /machine before first use" for all programs, not just qemu-system-FOO, but that might be more invasive. Not sure. > This is achieved with a new do_qdev_get_machine() function called container_get() is a suboptimal name for a function that creates containers, qdev_get_machine() is a suboptimal name for a function that creates /machine, and so is do_qdev_get_machine(). Observation, not demand. > from qdev_get_machine(), with different implementations for system > and user mode. > > $ ./qemu-system-ppc64 -device power8_v2.0-spapr-cpu-core,help > qemu-system-ppc64: ../../hw/core/machine.c:1290: > qdev_get_machine: Assertion `machine != NULL' failed. > Aborted (core dumped) > > Reported-by: Thomas Huth <th...@redhat.com> > Signed-off-by: Greg Kurz <gr...@kaod.org>