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    SubjectRe: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread
    ----- On Feb 27, 2016, at 1:35 PM, Linus Torvalds torvalds@linux-foundation.org wrote:

    > On Sat, Feb 27, 2016 at 6:58 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    >>
    >> Paul's patches have the following structure:
    >>
    >> struct thread_local_abi {
    >> union {
    >> struct {
    >> u32 cpu_id;
    >> u32 seq;
    >> };
    >> u64 cpu_seq;
    >> };
    >> unsigned long post_commit_ip;
    >> };
    >
    > Please don't do "unsigned long" in ABI structures any more.
    >
    > Make it u64, and make sure it is 64-bit aligned (which it would be in
    > this case). Make it so that we don't have to have separate compat
    > paths.

    AFAIU, this "post_commit_ip" field is expected to be updated
    with a single-copy-store by user-space. If we want to handle both
    32-bit and 64-bit processes, how do you recommend doing this
    without an unsigned long type ?

    A 64-bit integer would not be a single-copy store for
    32-bit processes, but a 32-bit integer would not be large
    enough for 64-bit processes.

    Would a

    union {
    uint32_t val32;
    uint64_t val64;
    } field;

    be an acceptable option ? Then the kernel could use
    one field or the other depending on the process bitness.

    Thanks,

    Mathieu

    --
    Mathieu Desnoyers
    EfficiOS Inc.
    http://www.efficios.com

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