Re: Difference between 'current_machine' vs MACHINE(qdev_get_machine())

[Like Xu <like.xu@linux.intel.com>]

On 2020/1/9 20:01, Paolo Bonzini wrote:
> On 09/01/20 12:23, Philippe Mathieu-Daudé wrote:
>>
>>
>>      current_machine =
>> MACHINE(object_new_with_class(OBJECT_CLASS(machine_class)));
>>      object_property_add_child(object_get_root(), "machine",
>>                                OBJECT(current_machine), &error_abort);
>>
>> The bigger user of 'current_machine' is the accel/KVM code.
>>
>> Recently in a0628599f..cc7d44c2e0 "Replace global smp variables with
>> machine smp properties" we started to use MACHINE(qdev_get_machine()).
>>
>> qdev_get_machine() resolves the machine in the QOM composition tree.
>> I am confused by this comment:
>>
>>    /* qdev_get_machine() can return something that's not TYPE_MACHINE
>>     * if this is one of the user-only emulators; in that case there's
>>     * no need to check the ignore_memory_transaction_failures board flag.
>>     */
>>
>> Following a0628599f..cc7d44c2e0, a5e0b33119 use 'current_machine' again.
>>
>> What are the differences between both form, when should we use one or
>> another (or can we use a single one?). Can this break user-only mode?
> 
> I would always use MACHINE(qdev_get_machine()), espeecially outside
> vl.c.  Ideally, current_machine would be static within vl.c or even
> unused outside the object_property_add_child() that you quote above.
> 
> Most of the times, I noticed from a quick grep, we actually want to
> access the accelerator, not the machine, so we could add a
> qemu_get_accelerator() wrapper that does
> MACHINE(qdev_get_machine())->accelerator.
> 
> Paolo
> 

I prefer to use MACHINE(qdev_get_machine()) wherever possible.

However, the qdev_get_machine() would return non TYPE_MACHINE object if:
- call qdev_get_machine() before we do 
"object_property_add_child(object_get_root(), "machine", 
OBJECT(current_machine), &error_abort);" in vl.c;
- or in the context with '#ifdef CONFIG_USER_ONLY';

Thanks,
Like Xu