Re: Rust high-level pre/post migration callbacks

[Paolo Bonzini <pbonzini@redhat.com>]

On 9/8/25 12:02, Manos Pitsidianakis wrote:
> Hi Paolo,
> 
> I'm not familiar with how migration works under the hood, but this
> data transformation design looks very clean and neat to me.

Thanks!

>> Using it is very easy, you can just declare your state like:
>>
>>       regs: Migratable<Mutex<MyDeviceRegisters>>
> 
> Hm it's a shame we cannot do this with a trait since it requires state
> storage for migration_state.

Yeah, but a Box is just a single pointer.  We need to have an actual 
location in memory for the C code to retrieve the pointer.

> A suggestion: we could declare a "mirror" struct to hold
> `migration_state` with a Derive macro. This is what the `rkyv` crate
> does with its `Archive` derive macro and trait
> <https://docs.rs/rkyv/latest/rkyv/trait.Archive.html>

Nice!  I'm not familiar with rkyv but it does look very similar.

When I tried to clean up the HPET (create HPETRegisters, and replace use 
of raw pointers with self_cell to implement self-referential structs), 
migration was the big mess, so I hope that this design will fix both 
that and Mutex<>.  And indeed having a derive macro is very similar to 
what I came up with one day later :) while thinking about how HPET could 
use it in practice.

Probably the derive macro would require some changes to the trait, but 
the basic idea remains the same.

Paolo