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

[Philippe Mathieu-Daudé]

On 4/15/21 3:30 PM, Greg Kurz wrote:
> On Thu, 15 Apr 2021 14:39:55 +0200
> Philippe Mathieu-Daudé <philmd@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 <thuth@redhat.com>
>>> Signed-off-by: Greg Kurz <groug@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...

> 
>> -- >8 --
>> --- a/hw/core/meson.build
>> +++ b/hw/core/meson.build
>> @@ -24,6 +24,8 @@
>>  common_ss.add(when: 'CONFIG_SPLIT_IRQ', if_true: files('split-irq.c'))
>>  common_ss.add(when: 'CONFIG_XILINX_AXI', if_true: files('stream.c'))
>>
>> +user_ss.add(files('qdev-user.c'))
>> +
>>  softmmu_ss.add(files(
>>    'fw-path-provider.c',
>>    'loader.c',
>> ---
>>
>> Thanks,
>>
>> Phil.
>>
>