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

[Greg Kurz]

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

> > 
> >> -- >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.
> >>
> > 
>