[RFC] Simplify type-level handling of vm_memory's Bitmap trait
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The `DescriptorChain::read` and `DescriptorChain::write` define a `B: BitmapSlice` type parameter. However, the type of the bitmap that will be associated with the `VolatileSlices` is already transitively prescribed by the `M` type parameter on the `DescriptorChain` object itself. To reconcile this, these functions need lengthy `WithBitmapSlice` bounds (which tells the compiler "these two different BitMap type variables are actually the same"). Drop the `B: Bitmap` slice parameter from these functions, and instead simply return `Reader<'a, MS<M::Target>>`, where `MS<M::Target>` resolves to the bitmap type induced by the guest memory implementation. This avoids the `WithBitmapSlice` bounds completely. Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
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Yep, I'm a bit worried too.
I think you should try the vhost-devices workspace where there are the devices that really consume the queues, maybe there we have more to change. |
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Summary of the PR
Hi vm-virtio folks!
I was messing around with vm-memory's bitmap tracking functionality, in hopes of making it less of a mess (commit in my fork: rust-vmm/vm-memory@ce80a91 - admittedly not sure if I succeeded), but in doing so, I found out why vm-virtio needs those
WithBitmapSlicetrait bounds. So this PR eliminates them (and has the added advantage of making vm-virtio forward compatible with my bitmap proposal to vm-memory).Admittedly, I'm a bit worried about all those explicit type hints the compiler suddenly needs in the tests (update: figured out what these are able:
Reader::writedoes not refer toSelfas a type at all now, so when I writeReader::<T>::new()the return value will be completely unrelated toT. ugh.) , but figured I'd throw it y'all's way anyway :)I compiled the vhost workspace against this, and everything goes through without modifications there
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