Re: [PATCH 1/2] qdev: Separate implementations of qdev_get_machine() for user and system

[Markus Armbruster]

Greg Kurz <groug@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 <thuth@redhat.com>
> Signed-off-by: Greg Kurz <groug@kaod.org>