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    SubjectRe: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods
    Peter Zijlstra wrote:
    > On Mon, 2009-06-15 at 14:59 -0400, Mathieu Desnoyers wrote:
    >> * Peter Zijlstra ( wrote:
    >>> On Mon, 2009-06-15 at 14:48 -0400, Mathieu Desnoyers wrote:
    >>>> we should not care that much about the performance hit of
    >>>> saving/restoring the cr2 register at each nmi entry/exit.
    >>> But we do, perf counters very much cares about nmi performance.
    >> To a point where it cannot afford a simple register save/restore ?
    >> There is "caring" and "_caring_". I am tempted to ask what NMI handler
    >> execution frequency you have in mind here to figure out if we are not
    >> trying to optimize sub-nanoseconds per minutes. ;)
    > Ah, well, I have no idea who expensive cr2 is, if its like a regular
    > register then it should be fine. If however its tons more expensive then
    > we should really avoid it.
    > As to the freq, 100kHz would be nice ;-)

    Writing control registers is serializing, so it's a lot more expensive
    than writing a normal register; my *guess* is that it will be on the
    order of 100-200 cycles.

    That is not based on any actual information.


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