On 4/15/21 6:56 PM, Greg Kurz wrote:
> On Thu, 15 Apr 2021 18:45:45 +0200
> Philippe Mathieu-Daudé <phi...@redhat.com> wrote:
>
>> On 4/15/21 3:30 PM, Greg Kurz wrote:
>>> On Thu, 15 Apr 2021 14:39:55 +0200
>>> Philippe Mathieu-Daudé <phi...@redhat.com> wrote:
>>>
>>>> On 4/9/21 6:03 PM, Greg Kurz wrote:
>>>>> 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 :
>>>>>
>>>>> 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).
>>>>>
>>>>> 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.
>>>>>
>>>>> This is achieved with a new do_qdev_get_machine() function called
>>>>> 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>
>>>>> ---
>>>>> hw/core/machine.c | 14 ++++++++++++++
>>>>> hw/core/qdev.c | 2 +-
>>>>> include/hw/qdev-core.h | 1 +
>>>>> stubs/meson.build | 1 +
>>>>> stubs/qdev-get-machine.c | 11 +++++++++++
>>>>> 5 files changed, 28 insertions(+), 1 deletion(-)
>>>>> create mode 100644 stubs/qdev-get-machine.c
>>>> ...
>>>>
>>>>> diff --git a/stubs/meson.build b/stubs/meson.build
>>>>> index be6f6d609e58..b99ee2b33e94 100644
>>>>> --- a/stubs/meson.build
>>>>> +++ b/stubs/meson.build
>>>>> @@ -54,3 +54,4 @@ if have_system
>>>>> else
>>>>> stub_ss.add(files('qdev.c'))
>>>>> endif
>>>>> +stub_ss.add(files('qdev-get-machine.c'))
>>>>
>>>> Adding this as a stub looks suspicious...
>>>> Why not add it in to user_ss in hw/core/meson.build?
>>>> Maybe name the new file hw/core/qdev-user.c?
>>>>
>>>
>>> It turns out that this isn't specific to user-mode but rather
>>> to any non-qemu-system-FOO binary built with qdev, e.g.
>>> test-qdev-global-props :
>>>
>>> #0 do_qdev_get_machine () at ../../stubs/qdev-get-machine.c:10
>>> #1 0x0000000100017938 in qdev_get_machine () at ../../hw/core/qdev.c:1134
>>> #2 0x000000010001855c in device_set_realized (obj=0x100128b60,
>>> value=<optimized out>, errp=0x7fffffffd4e0) at ../../hw/core/qdev.c:745
>>> #3 0x000000010001cc5c in property_set_bool (obj=0x100128b60, v=<optimized
>>> out>, name=<optimized out>, opaque=0x1000f33f0, errp=0x7fffffffd4e0) at
>>> ../../qom/object.c:2257
>>> #4 0x0000000100020a9c in object_property_set (obj=0x100128b60,
>>> name=0x100093f78 "realized", v=0x100136d30, errp=0x1000e3af8 <error_fatal>)
>>> at ../../qom/object.c:1402
>>> #5 0x000000010001c38c in object_property_set_qobject (obj=0x100128b60,
>>> name=0x100093f78 "realized", value=<optimized out>, errp=0x1000e3af8
>>> <error_fatal>) at ../../qom/qom-qobject.c:28
>>> #6 0x0000000100020e20 in object_property_set_bool (obj=0x100128b60,
>>> name=0x100093f78 "realized", value=<optimized out>, errp=0x1000e3af8
>>> <error_fatal>) at ../../qom/object.c:1472
>>> #7 0x000000010001612c in qdev_realize (dev=0x100128b60, bus=<optimized
>>> out>, errp=0x1000e3af8 <error_fatal>) at ../../hw/core/qdev.c:389
>>> #8 0x000000010000fb10 in test_static_prop_subprocess () at
>>> /home/greg/Work/qemu/qemu-master/include/hw/qdev-core.h:17
>>> #9 0x00007ffff7e95654 in g_test_run_suite_internal () from
>>> /lib64/libglib-2.0.so.0
>>> #10 0x00007ffff7e954b8 in g_test_run_suite_internal () from
>>> /lib64/libglib-2.0.so.0
>>> #11 0x00007ffff7e954b8 in g_test_run_suite_internal () from
>>> /lib64/libglib-2.0.so.0
>>> #12 0x00007ffff7e954b8 in g_test_run_suite_internal () from
>>> /lib64/libglib-2.0.so.0
>>> #13 0x00007ffff7e954b8 in g_test_run_suite_internal () from
>>> /lib64/libglib-2.0.so.0
>>> #14 0x00007ffff7e959cc in g_test_run_suite () from /lib64/libglib-2.0.so.0
>>> #15 0x00007ffff7e95a80 in g_test_run () from /lib64/libglib-2.0.so.0
>>> #16 0x000000010000ecec in main (argc=<optimized out>, argv=<optimized out>)
>>> at ../../tests/unit/test-qdev-global-props.c:316
>>>
>>> Is there a meson thingy to handle this dependency ?
>>
>> if not have_system
>> common_ss.add(files('qdev-machine-stubs.c'))
>> endif
>>
>> This is not pretty, but better than a generic stubs/qdev-get-machine.c
>> IMO...
>>
>
> Yeah it isn't that much different,
I'd expect symbols in stubs/ to be declared weak, while not with this
approach, so we'd catch clash for incorrect configs. Maybe I'm wrong...
> except maybe an improvement on the
> file location. Thanks for tip !
