My understanding matches yours, but it's worth noting that (IIUC) memory and PCIe are (last time I checked?) a separate I/O subsystem that just happens to resid

My understanding matches yours, but it's worth noting that (IIUC) memory and PCIe are (last time I checked?) a separate I/O subsystem that just happens to reside within the same package as the CPU on modern chips. So P2PDMA avoids burning CPU cycles and RAM bandwidth shuffling data around that you never wanted to use on the CPU anyway. (Also see: https://lwn.net/Articles/767281/ https://lwn.net/Articles/767281/)