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    SubjectRe: [perfmon] perfmon2 code review: 32-bit ABI on 64-bit OS

    On Sun, Feb 12, 2006 at 12:16:25AM +0600, Philip Mucci wrote:
    > >
    > > On some 64-bit arches (e.g. x86_64), most userspace code is 64-bit,
    > > while on others (e.g. powerpc), most is 32-bit. Reducing the number of
    > > things that a userspace tool or library writer can trip over seems like
    > > a good thing here, even if it slightly complicates perfmon's internals.
    > >
    > > > Note that there are similar issues with the remapped sampling buffer.
    > > > There, you need to explicitly compile your tool with a special option
    > > > to force certain types to be 64-bit (size_t, void *).
    > >
    > > It's pretty normal to just use 64-bit quantities in these cases, and
    > > cast appropriately.
    > I agree with Bryan. Stephane, do you have any quantitative data for how
    > much more expensive going to 64 bit quantities would be? Which
    > performance critical operations access this structure? AFAIK, any
    > performance monitoring system call is already slow by nature...and thus
    > an additional dozen cycles isn't going to make a difference. Of course,
    > if this structure needs to be read/written by get_pmd, including the
    > userspace version (+ mmap offset), then the extra overhead should be
    > considered.
    I think I can easily convert the bitmasks to be u64 on all platforms.
    I don't think it will negatively impact performance on 32-bit applications.

    The sampling buffer is another matter. It is directly remapped. The default
    format, exposes size_t and void *. The size_t is not on the critical
    path, it is used to specify the buffer size. If we expose as 64-bit,
    we need to check on 32-bit system that the value is below 4GB and cast
    to size_t.

    The most challenging piece is the IP (program pointer) that is in every
    sample. Today it is defined as unsigned long because this is fairly
    natural for a code address. The 64bit OS captures addresses as 64-bit,
    the 32-bit monitoring tool running on top has to consume them as 64-bit
    addresses, so u64 would be fine.

    But not on a 32-bit kernel with a 32-bit tool, addresses exported as u64
    would certainly work but consume double to buffer space, and that is a
    more serious issue in my mind.

    What we need is:
    1/ 32-bit OS: IP is 32-bit in the sampling buffer
    2/ 64-bit OS: IP is 64-bit in the sampling buffer

    Because of 32-bit ABI tool running on 2/, the IP would have
    to be defined as u64. But then it would be overkill on 1/.

    The problem is in the user level header file for the sampling buffer.
    We would need a data type that is 64-bit for IP if the host OS is 64-bit
    (regardless of the ABI used by the tool, i.e., the compiler). And a data
    type that is 32-bit on 32-bit OS. The problem is that there is no compiler
    flag or header flag somewhere that could guide the compiler. In the case
    of MIPS, we have defined a libpfm compile flags that indicates we want
    the 64-bit OS definition when compiling for a 32-bit application.

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