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

[Paolo Bonzini <pbonzini@redhat.com>]

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
>>
>>
> 
>