quoted

For my education, what is the API to send an IPI?
And the API to handle an IPI?

There are a few ways you could implement some custom cross-call,
although in this case I think stop_machine() would probably be the most
appropriate candidate. However, you're right that in general it may not
actually help enough to be worthwhile - a DSB SY would ensure that
in-flight transactions have at least been observed by the CPUs and any
other coherent masters, but for any writes with a memory type allowing
early acknowledgement (i.e. a Normal or Device mapping of a BAR) that
doesn't necessarily correlate with them having reached their ultimate
destination. For a PCI destination in particular, I think the normal way
to ensure all posted writes have completed would be to read from config
space; ah...

He almost certainly doesn't need to wait for the cycle to complete,
just long enough for the cycle to have been sent.

	David