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From: Like Xu <like.xu@linux.intel.com>
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Date: Tue, 2 Apr 2019 21:09:39 +0800
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Subject: Re: [Qemu-devel] [PATCH v2] vl.c: make current_machine as
 non-global variable
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To: Markus Armbruster <armbru@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Thomas Huth <thuth@redhat.com>, Eduardo Habkost <ehabkost@redhat.com>, qemu-trivial@nongnu.org, qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>, Igor Mammedov <imammedo@redhat.com>

On 2019/4/2 19:27, Markus Armbruster wrote:
> Like Xu <like.xu@linux.intel.com> writes:
> 
>> This patch makes the remaining dozen or so uses of the global
>> current_machine outside vl.c use qdev_get_machine() instead,
>> and then make current_machine local to vl.c instead of global.
>>
>> Signed-off-by: Like Xu <like.xu@linux.intel.com>
> 
> You effectively replace
> 
>      current_machine
> 
> by
> 
>      MACHINE(qdev_get_machine())
> 
> qdev_get_machine() uses container_get(), which has a side effect: any
> path component that doesn't exist already gets created as "container"
> object.  In case of qdev_get_machine(), that's just "/machine".
> 
> Creating "/machine" as "container" is of course wrong.  You therefore
> must not use qdev_get_machine() before main() creates "/machine".  It
> does like this:
> 
>      object_property_add_child(object_get_root(), "machine",
>                                OBJECT(current_machine), &error_abort);
> 
> I recently had several cases of code rearrangements explode because the
> reordered code called qdev_get_machine() too early.  Makes me rather
> skeptical about this patch.  To be frank, I consider qdev_get_machine()
> a trap for the unwary.  container_get(), too.
> 
> If we decide using it to make current_machine static a good idea anyway,
> we need to check the new uses carefully to make sure they can't run
> before main() creates "/machine".
> 
> 

Thanks for your comments and this issue may not exist in this patch.

I am curious when and where we would call qdev_get_machine() before 
main() initializes current_machine and adds it to QOM store which is
called very early.