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    SubjectRe: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods
    Linus Torvalds wrote:
    > On Mon, 15 Jun 2009, Ingo Molnar wrote:
    >> A simple cr2 corruption would explain all those cc1 SIGSEGVs and
    >> other user-space crashes i saw, with sufficiently intense sampling -
    >> easily.
    > Note that we could work around the %cr2 issue, since any corruption is
    > always nicely "nested" (ie there are never any SMP issues with async
    > writes to the register).
    > So what we _could_ do is to have a magic value for %cr2, along with a "NMI
    > sequence count", and if we see that value, we just return (without doing
    > anything) from the page fault handler.

    Wouldn't it be simpler to just require the NMI handler to save and
    restore %cr2 around any potentially faulting references?


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