Just thinking out loud: - what if we always way overallocate the swiotlb buffer - and then mark the second half / two thirds / <pull some number out of the thin air> slots as used, and make that region available through a special CMA mechanism as ZONE_MOVABLE (but not allowing other CMA allocations to dip into it). This allows us to have a single slot management for the entire area, but allow reclaiming from it. We'd probably also need to make this CMA variant irq safe. This could still be combined with more aggressive use of per-device swiotlb area, which is probably a good idea based on some hints. E.g. device could hint an amount of inflight DMA to the DMA layer, and if there are addressing limitations and the amout is large enough that could cause the allocation of a per-device swiotlb area.
