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    SubjectRe: Enabling RDPMC in user space by default
    Date
    On Tuesday 29 November 2005 09:15, Andi Kleen wrote:
    > Hallo,
    >
    > I'm considering to enable CR4.PCE by default on x86-64/i386. Currently it's
    > 0 which means RDPMC doesn't work. On x86-64 PMC 0 is always programmed to
    > be a cycle counter, so it would be useful to be able to access
    > this for measuring instructions. That's especially useful because RDTSC
    > does not necessarily count cycles in the current P state (already
    > the case on Intel CPUs and AMD's future direction seems to also
    > to decouple it from cycles) Drawback is that it stops during idle, but
    > that shouldn't be a big issue for normal measuring. It's not useful
    > as a real timer anyways.
    >
    > On Pentium 4 it also has the advantage that unlike RDTSC it's not
    > serializing so should be much faster.
    >
    > The kernel change would be to always set CR4.PCE to allow RDPMC
    > in ring 3.
    >

    You might also ping Stephane Eranian and the folks that are working on
    defining a common performance measurement interface in the kernel
    over on perfctr-devel@lists.sourceforge.net and see what they think.
    I'll cc them on this reply.

    > It would be actually a good idea to disable RDTSC in ring 3 too
    > (because user space usually doesn't have enough information to make
    > good use of it and gets it wrong), but I fear that will break
    > too many applications right now.
    >

    FWIW, I agree here. We lock down the power state and still use RDTSC for
    some timing things.

    > Any comments on this?
    >
    > -Andi
    >
    > -
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