Re: [PATCH 8/9] rust/hpet: Support migration

[Paolo Bonzini <pbonzini@redhat.com>]

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Il mer 16 apr 2025, 12:00 Zhao Liu <zhao1.liu@intel.com> ha scritto:

     > So the problem is that, in a "&'a Foo<T>", T must also be "T: 'a".
     > One solution is for vmsd() to return an
     > Option<VMStateDescription<Self>>, and do
    Box::into_raw(Box::new(vmsd))
     > in the class_init method. Once we have const_refs_static, "fn vmsd()"
     > can become a const and the Box is not needed anymore.

    Thanks so much, that's a good idea!

    About `Box::into_raw(Box::new(vmsd))`, do you think it's necessary
    to use
    Box::leak(Box::new(*))? (though the Box<> isn't actively dropped during
    the class's existence)


It's the same; leak and into_raw only differ in the return type. You can 
use leak if you prefer.

     > I attach the conversion I did of the other devices and tests. I
    am not
     > sure if it's possible to avoid having a huge patch to do
    everything at
     > once (except HPET since that can be added separately).

    Thank you again! From my initial thoughts: Splitting is also possible,
    but it requires first renaming VMStateDescription<T> to
    VMStateDescriptionWrapper<T>, then replacing it in pl011 and test (and
    hpet) one by one, and finally renaming it back to VMStateDescription<T>.
    If you prefer this approach, I can help you split your patch below.


Or maybe first you keepvmsd() return 
Option<&bindings::VMStateDescription> and stick a get_vmsd_ref() in "fn 
vmsd()", then in a final patch you make it return 
Option<VMStateDescription<T>> and introduce the Box::leak(Box::new(...)) 
trick.
> > + vmstate_struct!(HPETState, timers[0 .. num_timers], 
> &VMSTATE_HPET_TIMER, BqlRefCell<HPETTimer>, 
> HPETState::validate_num_timers).with_version_id(0),
>
> And it seems like you don't oppose the hack in patch 1? ;-)
It's not bad, but I haven't looked at why it's needed yet (i.e. why a 
constant function would be evaluating a destructor).

Paolo

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