Enabling RDPMC in user space by default

From: Andi Kleen (ak_at_suse.de)
Date: 11/29/05

  • Next message: Pavel Machek: "[PATCH] swsusp: Drop duplicate prototypes" 
    Date:	Tue, 29 Nov 2005 16:15:15 +0100
To: discuss@x86-64.org

    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.

    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.

    Any comments on this?

    -Andi

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