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    SubjectRe: [PATCH 1/3] perf events: fix WARN_ON_ONCE for 64-bit raw data, SW events


    On 02/18/11 07:55, Peter Zijlstra wrote:
    >> Another gotcha is in perf_output_sample there is:
    >> struct {
    >> u32 size;
    >> u32 data;
    >> } raw = {
    >> .size = sizeof(u32),
    >> .data = 0,
    >> };
    >>
    >> While that meets the 8-byte stride I was concerned that changing the
    >> size field to u64 breaks ABI.
    >
    > It would.
    >
    > No, what you need to do is provide data that is sized such that it
    > matches the 8b stride adding padding where needed.

    I want to push a 64-bit counter to userspace. By definition it meets the
    8-byte stride. Now how do I get sizeof(size field) + sizeof(data) to
    meet an 8-byte stride? I can't when sizeof(size field) = 4. Ok, I can
    pad the data with 4 bytes, but then I an api to tell the user to strip
    the 4-byte pad. Seems like an odd way to go.

    Why is an 8-byte stride preferred over a 4-bytes?

    David

    >
    > Anyway, I don't think you need RAW at all.
    >


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