Messages in this thread | | | Date | Sat, 20 Jun 2009 19:40:22 +0200 | From | Ingo Molnar <> | Subject | Re: Accessing user memory from NMI |
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* Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> * Ingo Molnar (mingo@elte.hu) wrote: > > > > * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > > > > > 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. > > > > Vegard raised the point that making NMIs pagefault-safe is also a > > plus for making kmemcheck NMI-safe. > > > > So besides it being faster (direct memory access versus 150 cycles > > GUP walk ... per frame entry!), it's also more robust in general. > > > > But too ambitious for v2.6.31 i think, unless patches become ready > > really soon. What we have right now is the 64-bit only and > > paravirt-unaware half-ported solution below. > > Side note: saving/restoring the cr2 register would additionally be > required around page faults in nmi handler in addition to the > patch below, both for 32 and 64-bits x86.
Correct, i just mentioned that in another mail - it is required to make sure we dont corrupt the main pagefault handler's cr2.
Ingo
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