On Sat, 10 Apr 2021 10:59:25 +0200 Markus Armbruster <arm...@redhat.com> wrote:
> 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. > Still armed and ready for subtle bugs. > > 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()? > No. I've only looked at the /machine case. > > 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 would mean all user emulation binaries and a bunch of test programs as well. I'll give a try in this direction. > > 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. > /** * container_get: * @root: root of the #path, e.g., object_get_root() * @path: path to the container * * Return a container object whose path is @path. Create more containers * along the path if necessary. * * Returns: the container object. */ Object *container_get(Object *root, const char *path); My understanding is that container_get()'s main mission is to return a "container" object. The creation part looks like a fallback to "fill the holes" in the QOM tree... I'd rather try to get rid of that side-effect entirely rather than coming up with a sensible name => auditing other users of container_get() as you asked above seems to be the next step :) Thanks! > > 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> >
