Accessing user memory from NMI

[Paul Mackerras <paulus@samba.org>]

What was the conclusion you guys came to about doing a user stack
backtrace in an NMI handler?  Are you going to access user memory
directly or are you going to use the __fast_get_user_pages approach?

Ben H and I were talking today about what we'd need in order to be
able to read user memory in a PMU interrupt handler.  It looks like we
could read user memory directly with a bit of care, on 64-bit at
least.  Because of the MMU hash table that would almost always work
provided the page has already been touched (which stack pages would
have been), but there is a small chance that the access might fail
even if the address has a valid PTE.  At that point we could fall back
to the __fast_get_user_pages method, but I'm not sure it's worth it.

Paul.