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

[Zhao Liu <zhao1.liu@intel.com>]

On Thu, Aug 07, 2025 at 04:13:00PM +0200, Paolo Bonzini wrote:
> Date: Thu, 7 Aug 2025 16:13:00 +0200
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: Re: [RFC 00/26] rust/memory: Integrate the vm-memory API from
>  rust-vmm
> 
> [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.

About Endianness, I have more thoughts in the reply of patch 24.

> 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?

with iommu support, the downstream patches are not necessary :-).

> > 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.

Thank you for your quick feedback.

> 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...)

Yes! For such a single function, there're lots of changes.

Thanks,
Zhao