Messages in this thread | | | Date | Thu, 15 Jul 2010 18:30:46 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [patch 1/2] x86_64 page fault NMI-safe |
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* Linus Torvalds (torvalds@linux-foundation.org) wrote: > 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.
There are 2 tests done on NMI handler entry:
1) test if nested over nmi_atomic region (which is a very restrained region around nmi_exit, which does not do any function call nor take traps). 2) test if the per-cpu nmi_nesting flag is set.
Test #2 takes care of NMIs nested over functions called and traps.
> > > This code assumes NMIs have a separate stack. > > It also needs to be made per-cpu (and the flags be per-cpu).
Sure, that was implied ;)
> > 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 could be used as a way to detect "nesting over NMI", but I'm not entirely sure it would deal with the "we need a fake NMI" flag set/clear (more or less equivalent to setting CS to 0 in your implementation and then back to some other value). The "set" is done with NMIs disabled, but the "clear" is done at fake NMI entry, where NMIs are active.
> > That would also avoid the race, because %rsp (as a flag) now gets > cleared atomically by the "iret". So that might actually solve things.
Well, I'm still unconvinced there is anything to solve, as I built my NMI entry with 2 tests: one for "nmi_atomic" code range and the other for per-cpu nesting flag. Given that I set/clear the per-cpu nesting flag either with NMIs off or within the nmi_atomic code range, this should all work fine.
Unless I am missing something else ?
Thanks,
Mathieu
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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