On Mon, Jun 14, 2010 at 07:36:10PM +1000, Benjamin Herrenschmidt wrote:

However, there's a lot of room for abuse here and I'm worried that if it
becomes widespread, we'll start seeing vendors use that as a way to do
some kind of HAL and hide various platform methods in there (clock
control, nvram, etc...).

This is what I'm worried about too.

As I said in my first reply in this thread, calling out from the kernel
will kill performance due to the time taken to shut down the caches and
MMU, which can only be done safely with all exceptions turned off.

The only time that it can be seriously considered is if you're calling
out to reboot, shutdown or kexec.