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    SubjectRe: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

    On Mon, 15 Jun 2009, tip-bot for Peter Zijlstra wrote:
    > __copy_from_user_inatomic() isn't NMI safe in that it can trigger
    > the page fault handler which is another trap and its return path
    > invokes IRET which will also close the NMI context.

    That's not the only problem.

    An even more fundamental problem is that the page fault handler is not
    re-entrant because of simple the value in %cr2. So regardless of any
    'iret' issues, you *CANNOT* take a page fault in an NMI, because the NMI
    might happen while we're in the critical region of having taken another
    page fault, but before we've saved off the value of %cr2 in that old page

    If the NMI handler causes a page fault, it will corrupt the %cr2 of the
    outer page fault. That's why the page fault is done with an interrupt
    gate, and why we have that conditional local_irq_enable() in it.

    So page faults are fundamentally only safe wrt normal interrupts, not NMI.


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