Re: [PATCH 13/15] rust: memory: wrap MemoryRegion with Opaque<>

[Paolo Bonzini]

On 2/25/25 10:14, Zhao Liu wrote:
> >   impl MemoryRegion {
> >       // inline to ensure that it is not included in tests, which only
> > @@ -174,13 +174,15 @@ pub fn init_io<T: IsA<Object>>(
> >           size: u64,
> >       ) {
> >           unsafe {
> > -            Self::do_init_io(&mut self.inner, owner.cast::<Object>(), &ops.0, 
> > name, size);
> > +            Self::do_init_io(
> > +                self.0.as_mut_ptr(),
>
> I'm not sure why the wrapper doesn't work here.
>
> If I change this to `self.as_mut_ptr()`, then compiler tries to apply
> `ObjectDeref::as_mut_ptr` and complains:
>
> the trait `qom::ObjectType` is not implemented for `bindings::MemoryRegion`

It's because the implementation of ObjectDeref for "&mut MemoryRegion" 
wins over coercing "&mut MemoryRegion" to "&MemoryRegion" and then 
calling MemoryRegion::as_mut_ptr().

I added a comment

        // self.0.as_mut_ptr() needed because Rust tries to call
        // ObjectDeref::as_mut_ptr() on "&mut Self", instead of coercing
        // to "&Self" and then calling MemoryRegion::as_mut_ptr().
        // Revisit if/when ObjectCastMut is not needed anymore; it is
        // only used in a couple places for initialization.

Paolo

> But when I modify the function signature to &self like:
>
> diff --git a/rust/qemu-api/src/memory.rs b/rust/qemu-api/src/memory.rs
> index fdb1ea11fcf9..a82348c4a564 100644
> --- a/rust/qemu-api/src/memory.rs
> +++ b/rust/qemu-api/src/memory.rs
> @@ -167,7 +167,7 @@ unsafe fn do_init_io(
>       }
>
>       pub fn init_io<T: IsA<Object>>(
> -        &mut self,
> +        &self,
>           owner: *mut T,
>           ops: &'static MemoryRegionOps<T>,
>           name: &'static str,
>
> Then the Wrapper's as_mut_ptr() can work.
>
> > +                owner.cast::<Object>(),
> > +                &ops.0,
> > +                name,
> > +                size,
> > +            );
> >           }
> >       }
> > -
> > -    pub(crate) const fn as_mut_ptr(&self) -> *mut bindings::MemoryRegion {
> > -        addr_of!(self.inner) as *mut _
> > -    }
> >   }
> >   
> >   unsafe impl ObjectType for MemoryRegion {
> > --
> > 2.48.1