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 <[email protected]>
> Signed-off-by: Greg Kurz <[email protected]>
> ---
> 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?
-- >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.
