Re: Accessing user memory from NMI

From: Peter Zijlstra
Date: Thu Jun 18 2009 - 04:36:22 EST

On Thu, 2009-06-18 at 18:20 +1000, Paul Mackerras wrote:
> 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.

Currently we have the GUP based approach, but Ingo is thikning about
making the pagefault handler NMI safe on x86 for .32.

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