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    SubjectRe: [patch 1/2] x86_64 page fault NMI-safe
    * 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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