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

From: Philippe Mathieu-Daudé
Subject: Difference between 'current_machine' vs MACHINE(qdev_get_machine())
Date: Thu, 9 Jan 2020 12:23:01 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2


"hw/boards.h" declare current_machine, and vl.c defines it:

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?



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