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Re: Crashes with qemu-system-ppc64

From: Greg Kurz
Subject: Re: Crashes with qemu-system-ppc64
Date: Wed, 24 Mar 2021 00:00:56 +0100

Cc'ing David

On Tue, 23 Mar 2021 17:48:36 +0100
Thomas Huth <thuth@redhat.com> wrote:

> In case anyone is interested in fixing those, there are two regressions with 
> qemu-system-ppc64 in the current master branch:
> $ ./qemu-system-ppc64 -M ppce500 -device macio-oldworld
> qemu-system-ppc64: ../../devel/qemu/softmmu/memory.c:2443: 
> memory_region_add_subregion_common: Assertion `!subregion->container' failed.
> $ ./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)

I've bisected this one to:

3df261b6676b5850e93d6fab3f7a98f8ee8f19c5 is the first bad commit
commit 3df261b6676b5850e93d6fab3f7a98f8ee8f19c5
Author: Peter Maydell <peter.maydell@linaro.org>
Date:   Fri Mar 13 17:24:47 2020 +0000

    softmmu/vl.c: Handle '-cpu help' and '-device help' before 'no default 
    Currently if you try to ask for the list of CPUs for a target
    architecture which does not specify a default machine type
    you just get an error:
      $ qemu-system-arm -cpu help
      qemu-system-arm: No machine specified, and there is no default
      Use -machine help to list supported machines
    Since the list of CPUs doesn't depend on the machine, this is
    unnecessarily unhelpful. "-device help" has a similar problem.
    Move the checks for "did the user ask for -cpu help or -device help"
    up so they precede the select_machine() call which checks that the
    user specified a valid machine type.
    Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

 softmmu/vl.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)
bisect run success

This change is fine but it unveils a bad assumption.

0  0x00007ffff64a3708 in raise () at /lib64/power9/libc.so.6
#1  0x00007ffff6483bcc in abort () at /lib64/power9/libc.so.6
#2  0x00000001008db940 in object_dynamic_cast_assert
    (obj=0x10126f670, typename=0x100c20380 "machine", file=0x100b34878 
"/home/greg/Work/qemu/qemu-ppc/include/hw/boards.h", line=<optimized out>, 
func=0x100bcd320 <__func__.30338> "MACHINE") at ../../qom/object.c:883
#3  0x0000000100456e00 in MACHINE (obj=<optimized out>) at 
#4  0x0000000100456e00 in cpu_core_instance_init (obj=0x10118e2c0) at 
#5  0x00000001008d9f44 in object_init_with_type (obj=obj@entry=0x10118e2c0, 
ti=0x1011fd470) at ../../qom/object.c:375
#6  0x00000001008d9f24 in object_init_with_type (obj=obj@entry=0x10118e2c0, 
ti=0x101211ad0) at ../../qom/object.c:371
#7  0x00000001008d9f24 in object_init_with_type (obj=obj@entry=0x10118e2c0, 
ti=ti@entry=0x101212760) at ../../qom/object.c:371
#8  0x00000001008dc474 in object_initialize_with_type 
(obj=obj@entry=0x10118e2c0, size=size@entry=160, type=type@entry=0x101212760) 
at ../../qom/object.c:517
#9  0x00000001008dc678 in object_new_with_type (type=0x101212760) at 
#10 0x00000001009fbad8 in qmp_device_list_properties (typename=<optimized out>, 
errp=<optimized out>) at ../../qom/qom-qmp-cmds.c:146
#11 0x00000001005a4bf0 in qdev_device_help (opts=0x10126c200) at 
#12 0x0000000100760afc in device_help_func (opaque=<optimized out>, 
opts=<optimized out>, errp=<optimized out>) at ../../softmmu/vl.c:1204
#13 0x0000000100ad1050 in qemu_opts_foreach (list=<optimized out>, 
func=0x100760ae0 <device_help_func>, opaque=0x0, errp=0x0) at 
#14 0x00000001007653cc in qemu_process_help_options () at 
#15 0x00000001007653cc in qemu_init (argc=<optimized out>, argv=<optimized 
out>, envp=<optimized out>) at ../../softmmu/vl.c:3521
#16 0x00000001002f4f88 in main (argc=<optimized out>, argv=<optimized out>, 
envp=<optimized out>) at ../../softmmu/main.c:49

Basically, "-device power8_v2.0-spapr-cpu-core,help" ends up
instantiating an object of the "power8_v2.0-spapr-cpu-core" type,
which derives from "cpu-core". The "cpu-core" type has an instance
init function that assumes that qdev_get_machine() returns an object
of type "machine"...

static void cpu_core_instance_init(Object *obj)
    MachineState *ms = MACHINE(qdev_get_machine());

qdev_get_machine() cannot return a valid machine type since
select_machine() hasn't been called yet... an instance init
function is probably not the best place to use qdev_get_machine()
if any.

    CPUCore *core = CPU_CORE(obj);

    core->nr_threads = ms->smp.threads;

It seems that this should rather sit in a device realize function,
when the machine type is known.

>   Thomas

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