[$] The trouble with 64-bit DMA

We live in a 64-bit world, to the point that many distributors want to stopsupporting 32-bit systems at all. However, lurking within our 64-bitkernels is a subsystem that has not really managed to move past 32-bitaddresses. The quick merge-window failure of an attempt to use64-bit addresses in the I/O memory-management unit (IOMMU) subsystem showshow hard it can be to leave all of one's 32-bit history behind.