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SubjectRe: [PATCH] perfcounters: fix type/event_id layout on big-endian systems

* Paul Mackerras <paulus@samba.org> wrote:

> Ingo Molnar writes:
>
> > hm, this ifdef really looks ugly. How about just changing event_id
> > to 64 bits and having a separate u32 type field? The size impact is
> > minimal, the cleanliness win is significant :-)
>
> We wanted to get a complete and unique identifier for the event
> into 64 bits so that we could put it into the ring buffer for
> PERF_RECORD_GROUP and have it take up only one 8-byte slot and yet
> identify uniquely which counter's value follows it. I don't know
> that that is absolutely necessary but it sounds like a nice
> property.
>
> We could easily go back to a 1-bit raw field and have the type be
> either a 64-bit raw value or an 8-bit type plus 32 or 56-bit
> event_id.
>
> Or we could keep the current layout but use explicit shifts and
> masks rather than bitfields.
>
> I don't know the current C rules concerning unions very well, but
> I have the impression that writing to one member of a union and
> reading another is undefined behaviour, which is another strike
> against the current code if true...
>
> Anyway, the point is that the current code doesn't compile on
> powerpc and wouldn't work properly even if it did, so we need to
> do something.

yeah - i pulled your fix, thanks Paul.

Maybe the best option is to get rid of the bitfields and use masks
...

Ingo


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