On 5/20/25 18:23, Zhao Liu wrote:
HPET does some very simple memory accesses; a good safe solution
for this may be the ``vm-memory`` crate. While I have not looked into
using it, ``vm-memory`` and ``vm-virtio`` were written with QEMU's
use cases in mind.
I'm working on this and trying to wrap simple memory access by
vm-memory.
Ok. Note that while the GuestAddressSpace corresponds QEMU's
AddressSpace (so far so good :)), QEMU's MemoryRegion is completely
unrelated to vm-memory's GuestMemoryRegion. That's because vm-memory
only operates on an array of non-overlapping regions, like QEMU's
FlatRange or MemoryRegionSection structs.
The GuestMemory (GuestAddressSpace::M) corresponds to QEMU's FlatView.
Indeed the functions in the trait match with what you expect of a FlatView:
fn num_regions(&self) -> usize;
fn find_region(&self, addr: GuestAddress) -> Option<&Self::R>;
fn iter(&self) -> impl Iterator<Item = &Self::R>;
If the GuestMemory is a FlatView, the GuestAddressSpace::T, implements
Clone + Deref<Target = FlatView>. It's not too hard to see that
GuestAddressSpace's memory() method must call
address_space_get_flatview() and the GuestAddressSpace::T's drop method
must call flatview_unref(). Let's call this (Rust-specific) struct
FlatViewRefGuard, or something like that.
Going back to the GuestMemoryRegion (<FlatView as GuestMemory>::R), it
could be either a QEMU FlatRange or a MemoryRegionSection. Neither are
good options. Without a MemoryRegionSection you can't support IOMMU
regions; but flatview_do_translate() returns the MemoryRegionSection by
value, and GuestMemory's
fn find_region(&self, addr: GuestAddress) -> Option<&Self::R>;
wants a reference instead!
Anyhow, all three types (AddressSpace, FlatView, FlatRange) are better
wrapped with Opaque.
Looking more at FlatRange, these are easy:
// Required methods
fn len(&self) -> GuestUsize;
fn start_addr(&self) -> GuestAddress;
But this one is another problem:
fn bitmap(&self) -> &Self::B;
because it returns the "Bitmap" by reference. QEMU's bitmap is a global
variable indexed by ram_addr_t. It would be better if this was declared
like this:
fn bitmap(&'a self) ->
<Self::B as WithBitmapSlice<'a>>::S
I have no idea if this can be changed in upstream vm-virtio. For now
maybe you can leave it as (). That's buggy but it's ok for a proof of
concept.
So... not sure what to do there. It seems like vm-memory is very close
to being usable by QEMU, but maybe not completely. :(
Paolo
