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

[Greg Kurz]

On Thu, 15 Apr 2021 21:07:33 +0200
Philippe Mathieu-Daudé <philmd@redhat.com> wrote:

> On 4/15/21 6:56 PM, Greg Kurz wrote:
> > On Thu, 15 Apr 2021 18:45:45 +0200
> > Philippe Mathieu-Daudé <philmd@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 <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...
> >>
> > 
> > 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...
> 

Ah yes, you're certainly right. I'm convinced :)

> > except maybe an improvement on the
> > file location. Thanks for tip !
>