Draft: Add virtio balloon #1995
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The CI script requires it to be in our source tree, so we should ignore it.
The split virtqueue implementation previously still used a Box around the token ring items. The array the token ring is based on is howevery already on the heap (in another Box). To avoid having to allocate for individual items, only allocating the ring array up-front, remove the Box around the items. To successfully recover memory from the balloon device in an OOM situation, virtqueue submission for a deflate request must not allocate. During OOM handling the global allocator is already locked, so this would deadlock.
Talc previously did not support allocations without attempting to use the OOM handler to recover. For the balloon device to allocate pages for the balloon however we want to just probe for and then allocate a chunk of pages. If the allocation fails we don't want to recover old memory, we just want to allocate a smaller chunk (since recovery with the balloon will be shrinking the balloon, causing us to uselessly shuffle around pages, possibly even failing). See SFBdragon/talc#43 which is the PR for merging this modification upstream. Another API function may be added for querying the current largest possible new allocation in the future. In that case the balloon driver code should be changed to use that instead of probing allocatable size in powers of two.
Talc can optionally collect and expose some basic statistics about allocations. Provide a feature that regularly prints these statistics via another executor tassk.
TODO: explanatory commit message
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Description and more explanatory comments are to follow