Re: [RFC 10/26] subprojects/vm-memory: Patch vm-memory for QEMU memory backend

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

(+Hanna: I would like to align with Hanna on 0002.diff patch :-))

On Thu, Aug 07, 2025 at 03:59:26PM +0200, Paolo Bonzini wrote:
> Date: Thu, 7 Aug 2025 15:59:26 +0200
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: Re: [RFC 10/26] subprojects/vm-memory: Patch vm-memory for QEMU
>  memory backend
> 
> On 8/7/25 14:30, Zhao Liu wrote:
> > Add 2 patches to support QEMU memory backend implementation.
> > 
> > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> > ---
> >   .../packagefiles/vm-memory-0.16-rs/0001.diff  |  81 +++++++++++++
> >   .../packagefiles/vm-memory-0.16-rs/0002.diff  | 111 ++++++++++++++++++
> >   subprojects/vm-memory-0.16-rs.wrap            |   2 +
> >   3 files changed, 194 insertions(+)
> >   create mode 100644 subprojects/packagefiles/vm-memory-0.16-rs/0001.diff
> >   create mode 100644 subprojects/packagefiles/vm-memory-0.16-rs/0002.diff
> > 
> > diff --git a/subprojects/packagefiles/vm-memory-0.16-rs/0001.diff b/subprojects/packagefiles/vm-memory-0.16-rs/0001.diff
> > new file mode 100644
> > index 000000000000..037193108d45
> > --- /dev/null
> > +++ b/subprojects/packagefiles/vm-memory-0.16-rs/0001.diff
> > @@ -0,0 +1,81 @@
> > +From 298f8ba019b2fe159fa943e0ae4dfd3c83ee64e0 Mon Sep 17 00:00:00 2001
> > +From: Zhao Liu <zhao1.liu@intel.com>
> > +Date: Wed, 6 Aug 2025 11:31:11 +0800
> > +Subject: [PATCH 1/2] guest_memory: Add a marker tarit to implement
> > + Bytes<GuestAddress> for GuestMemory
> 
> This was a bit surprising.  Maybe this is something where GuestMemory needs
> some extra flexibility.

At least, the default GuestMemory::try_access() need to re-implement in
QEMU, and this is because GuestMemory::iter() doesn't fit for QEMU's
case, and GuestMemory::to_region_addr() also needs adjustment to support
complete translation.

For details,

1) iter() - QEMU has implemented the two-level "page" walk in
   `phys_page_find`, which is more efficient than linear iteration.

2) to_region_addr() - it's function signature is:

    fn to_region_addr(
        &self,
	addr: GuestAddress
    ) -> Option<(&Self::R, MemoryRegionAddress)>;

but QEMU currentlt wants:

    fn translate(
        &self,
        addr: GuestAddress,
        len: GuestUsize,
        is_write: bool,
    ) -> Option<(&MemoryRegionSection, MemoryRegionAddress, GuestUsize)>

`is_write` is mainly about IOMMU (and read-only case, but that could be
workaround I think).

And the 3rd member `GuestUsize` of (&MemoryRegionSection,
MemoryRegionAddress, GuestUsize) indicates the remianing size, which is
used to detect cross-region case. Maybe this `GuestUsize` is not
necessary in its return, since we can check the size of `MemoryRegionSection`
later. But this would be a bit repetitive.

But at least, this marker trait is acceptable, right? :-)

The marker trait for GuestMemoryRegion is introduced at commit 66ff347
("refactor: use matches! instead of to_string() for tests").

> > @@ -0,0 +1,111 @@
> > +From 2af7ea12a589fde619690e5060c01710cb6f2e0e Mon Sep 17 00:00:00 2001
> > +From: Zhao Liu <zhao1.liu@intel.com>
> > +Date: Wed, 6 Aug 2025 14:27:14 +0800
> > +Subject: [PATCH 2/2] guest_memory: Add is_write argument for
> > + GuestMemory::try_access()
> 
> This should be fine.  But Hanna is also working on IOMMU so maybe this won't
> be needed!

I'm not sure what method could align with Hanna's design. If there's
another interface/method, I can have a try.

Or, should I just ignore all IOMMU code path directly? This may need
to decouple some C codes. For example, I can split flatview_do_translate
into a non-iommu case and an iommu case.

Thanks,
Zhao