Messages in this thread | | | Date | Thu, 15 Jul 2010 15:16:12 -0700 | Subject | Re: [patch 1/2] x86_64 page fault NMI-safe | From | Linus Torvalds <> |
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On Thu, Jul 15, 2010 at 3:01 PM, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote: > > . NMI exit code > and fake NMI entry are made reentrant with respect to NMI handler interruption > by testing, at the very beginning of the NMI handler, if a NMI is nested over > the whole nmi_atomic .. nmi_atomic_end code region.
That is totally bogus. The NMI can be nested by exceptions and function calls - the whole _point_ of this thing. So testing "rip" for anything else than the specific final "iret" is meaningless. You will be in an NMI region regardless of what rip is.
> This code assumes NMIs have a separate stack.
It also needs to be made per-cpu (and the flags be per-cpu).
Then you could in fact possibly test the stack pointer for whether it is in the NMI stack area, and use the value of %rsp itself as the flag. So you could avoid the flag entirely. Because testing %rsp is valid - testing %rip is not.
That would also avoid the race, because %rsp (as a flag) now gets cleared atomically by the "iret". So that might actually solve things.
Linus
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