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SubjectAccessing user memory from NMI
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.


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