From: Ben Herrenschmidt<address@hidden>
Usually, PAPR virtual IO devices use a virtual IOMMU mechanism, TCEs,
to mediate all DMA transfers. While this is necessary for some sorts of
operation, it can be complex to program and slow for others.
This patch implements a mechanism for bypassing TCE translation, treating
"IO" addresses as plain (guest) physical memory addresses. This has two
main uses:
* Simple, but 64-bit aware programs like firmwares can use the VIO devices
without the complexity of TCE setup.
* The guest OS can optionally use the TCE bypass to improve performance in
suitable situations.
The mechanism used is a per-device flag which disables TCE translation.
The flag is toggled with some (hypervisor-implemented) RTAS methods.