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

From: Paolo Bonzini
Subject: Re: Difference between 'current_machine' vs MACHINE(qdev_get_machine())
Date: Thu, 9 Jan 2020 13:01:30 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1

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


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