Post Syndicated from original https://lwn.net/Articles/889593/

In theory, direct memory access (DMA) operations are simple to understand;
a device transfers data directly to or from a memory buffer managed by the
CPU. Almost all contemporary devices perform DMA, since it would not be
possible to obtain the needed performance without it. Like so many things,
DMA turns out to be a bit more complicated in practice. That complexity
led to an erroneous patch, intended to improve security, breaking DMA for
some devices in 5.17 and some stable kernels.