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    SubjectRe: [PATCH] perf_counter: extensible perf_counter_attr
    Ingo Molnar wrote:
    > * Corey Ashford <cjashfor@linux.vnet.ibm.com> wrote:
    >
    >
    >> If I understand you correctly, you would simply make
    >> perf_counter_attr larger every time you want to add a new
    >> attribute. Users using the new attributes would call
    >> sys_perf_counter_open with a larger attr_size value.
    >>
    >
    > Yes, exactly. Basically ABIs in Linux only get extended (never
    > shrunk and never changed) so it's not like we ever want to (or can)
    > shrink the size of the structure or change its semantics.
    >
    > Each future extension gives the structure a new, unique size - which
    > also acts as an 'ABI version' identifier, in a pretty robust way. We
    > check this 'ABI version' (the structure size) in the kernel code so
    > it's not just a passive 'version field' thing.
    >
    > Here are the various compatibility variations:
    >
    > - same-version kernel and user-space: they both use the same
    > attr_size value and support the full set of features.
    >
    > - old user-space running on new kernel: works fine, as the kernel
    > will do a short copy and zero out the remaining attributes.
    >
    > - new user-space running on old kernel: the kernel returns -ENOTSUP
    > and user-space has a choice to refuse to run cleanly - or, if an
    > old ABI version is widespread, might chose to utilize the old,
    > smaller attribute structure size field (at the cost of not using
    > new attribute features, obviously).
    >
    > ( Additional detail: in the size mismatch failure case the kernel
    > should write back the supported size into attr_size, so that
    > user-space knows which precise ABI variant it deals with on the
    > kernel side. )
    >
    > This kind of ABI maintenance method has a number of substantial
    > advantages:
    >
    > - It is very compatible (see above)
    >
    > - It is extensible easily and in an unlimited way - we just extend
    > the structure size.
    >
    > - It is very clean on the kernel side and the user side as well,
    > because we just have a single attribute structure.
    >
    > - It makes the ABI 'version' field an _active_ component of
    > functionality - so there is no way for subtle breakages to slip
    > in.
    >
    > - New attributes are prime-time members of the attribute structure,
    > not second-class citizens that first have to be read in via an
    > elaborate chaining mechanism at extra cost.
    >
    > [ If only all our syscall ABIs used this technique :-) It would be
    > so much easier to extend syscalls cleanly - without having to go
    > through the expensive and time-consuming process to add new
    > syscalls. ]
    >
    Thanks for the detailed explanation :)

    >> What about arch-dependent attributes? Would you want to place
    >> them all in the perf_counter_attr struct? I suppose this could be
    >> done by #include'ing an arch-specific .h file.
    >>
    >
    > What arch-dependent attributes are you thinking about? In the
    > perfcounters subsystem we want to support PMU and other performance
    > analysis features in a way that makes it possible for all
    > architectures to make use of them.
    >
    > So 'arch dependent attributes' per se are bad and against the
    > perfcounters design. "Generic perfcounter feature only supported by
    > a single architecture initially" is better.
    >
    Well, I think Intel has PEBS and AMD has some similar mechanism. I
    would guess that at some point you would want to provide access to those
    PMU features via these attributes. Since these mechanisms are very
    chip-specific, I don't think you would want to try to create an
    arch-independent interface to them. There may be future mechanisms that
    only make sense on one particular chip design, and would therefore not
    be a candidate for wider use, but would still make sense to provide some
    support for that mechanism via the attributes.

    Did you have some different plan for PEBS (etc.) ?

    - Corey



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