Re: [RFC 00/26] rust/memory: Integrate the vm-memory API from rust-vmm

[Paolo Bonzini <pbonzini@redhat.com>]

[Adding Hanna who's been working on vm-memory]

On 8/7/25 14:30, Zhao Liu wrote:
> Hi,
> 
> This RFC series explores integrating the vm-memory API into QEMU's
> rust/memory bindings.
> 
> Thanks to Paolo and Manos's many suggestions and feedback, I have
> resolved many issues over the past few months, but there are still
> some open issues that I would like to discuss.
> 
> This series finally provides the following safe interfaces in Rust:
>   * AddressSpace::write in Rust <=> address_space_write in C
>     - **but only** supports MEMTXATTRS_UNSPECIFIED
> 
>   * AddressSpace::read in Rust <=> address_space_read_full in C
>     - **but only** supports MEMTXATTRS_UNSPECIFIED.
> 
>   * AddressSpace::store in Rust <=> address_space_st{size} in C
>     - **but only** supports MEMTXATTRS_UNSPECIFIED and native endian.
> 
>   * AddressSpace::load in Rust <=> address_space_ld{size} in C
>     - **but only** supports MEMTXATTRS_UNSPECIFIED and native endian.

Endianness can be handled by BeNN and LeNN.  For MemTxAttrs we can use 
Bytes<(GuestAddress, MemTxAttrs)> (a variant on something you mention 
below).

Thinking out loud: maybe if we do our implementation in 
Bytes<(GuestAddress, MemTxAttrs)>, and Bytes<GuestAddress>::try_access 
wraps Bytes<(GuestAddress, MemTxAttrs)>, your downstream-only changes 
are not needed anymore?

> And this series involves changes mainly to these three parts:
>   * NEW QEMU memory APIs wrapper at C side.
>   * Extra changes for vm-memory (downstream for now).
>   * NEW QEMU memory bindings/APIs based on vm-memory at Rust side.
> 
> Although the number of line changes appears to be significant, more
> than half of them are documentation and comments.
Yep, thanks for writing them.

This is a good RFC, it's complete enough to show the challenges and the 
things that are missing stand up easily.

I'll look into what vm-memory is missing so that we can simplify QEMU's 
code further, but the basic traits match which is nice.  And the final 
outcome, which is essentially:

     let (addr, value) = (GuestAddress(self.fsb >> 32),
                          Le32(self.fsb as u32));
     ADDRESS_SPACE_MEMORY.memory().store(addr, value);

is as clean as it can be, if anything a bit wordy due to the 
GuestAddress "newtype" wrapper.  (If we decide it's too bad, the 
convenience methods in AddressSpace can automatically do the 
GuestAddress conversion...)

Paolo