Re: [Perfctr-devel] Re: Enabling RDPMC in user space by default

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

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    Date:	Tue, 29 Nov 2005 23:43:47 +0100
To: Nicholas Miell <nmiell@comcast.net>

    > I think you really need to come up with a better justification than "I
    > think this will be useful" for a permanent user-space ABI change.

    There's no user space ABI change involved, at least not from
    the kernel side. Hardware is breaking some assumptions people
    made though (they actually never worked fully, but these days they
    break more clearly) and this is a best effort to adapt.

    To give an bad analogy RDTSC usage in the last years is
    like explicit spinning wait loops for delays in the earlier
    times. They tended to work on some subset of computers,
    but were always bad and caused problems and people eventually learned
    it was better to use operating system services for this.

    The kernel will probably not disable RDTSC outright,
    but will make it clear in documentation that it's a bad
    idea to use directly and laugh at everybody who runs
    into problems with it.

    oprofile usage might change slightly though, although only
    for a small subset of its users. There can't be very many
    of them using multiple performance counters anyways because
    at least in the last 0.9 release I tried it didn't even work.

    > What problem are you trying to solve, why is that a problem, how does
    > making PMC0 always be a cycle counter solve that, what makes you think

    Read the original messages in the thread. They explain it all.

    > that future CPUs will have the same type of cycle counter that behaves
    > the same way as the current cycle counters, etc.
    >
    > AFAICT, the problem you're trying to solve is two-fold:
    >
    > a) RDTSC is serializing and RDPMC isn't.
    >
    > Which is nicely solved by RDTSCP.

    No, you got that totally wrong. Please read the RDTSCP specification again.

    > and
    > b) RDTSC isn't well defined.

    It's well defined - but in a way that makes it useless for cycle
    measurements these days.

    >
    > Well, RDPMC isn't defined at all. You're assuming that future processor
    > revisions will have the same or substantially similar PerfCtrs as
    > current processors, and nothing guarantees that at all.

    Point, but i guess it is reasonable to assume that future x85 CPUs
    will have cycle counter perfctrs. I cannot imagine anybody dropping
    such a basic facility.

    -Andi

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