perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail}

When perf runs in compat mode (kernel in 64-bit mode and the perf is in
32-bit mode), the 64-bit value atomicity in the user space cannot be
assured, E.g. on some architectures, the 64-bit value accessing is split
into two instructions, one is for the low 32-bit word accessing and
another is for the high 32-bit word.

This patch introduces weak functions compat_auxtrace_mmap__read_head()
and compat_auxtrace_mmap__write_tail(), as their naming indicates, when
perf tool works in compat mode, it uses these two functions to access
the AUX head and tail.  These two functions can allow the perf tool to
work properly in certain conditions, e.g. when perf tool works in
snapshot mode with only using AUX head pointer, or perf tool uses the
AUX buffer and the incremented tail is not bigger than 4GB.

When perf tool cannot handle the case when the AUX tail is bigger than
4GB, the function compat_auxtrace_mmap__write_tail() returns -1 and
tells the caller to bail out for the error.

These two functions are declared as weak attribute, this allows to
implement arch specific functions if any arch can support the 64-bit
value atomicity in compat mode.

Suggested-by: Adrian Hunter <[email protected]>
Signed-off-by: Leo Yan <[email protected]>
Acked-by: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: John Garry <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Poirier <[email protected]>
Cc: Mike Leach <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: "Russell King (oracle)" <[email protected]>
Cc: Suzuki Poulouse <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lore.kernel.org/lkml/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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