commit 5b590160d2cf776b304eb054afafea2bd55e3620 upstream.

If the trace data buffer becomes full, a truncated flag [T] is reported
in PERF_RECORD_AUX.  In some cases, the size reported is 0, even though
data must have been added to make the buffer full.

That happens when the buffer fills up from empty to full before the
Intel PT driver has updated the buffer position.  Then the driver
calculates the new buffer position before calculating the data size.
If the old and new positions are the same, the data size is reported
as 0, even though it is really the whole buffer size.

Fix by detecting when the buffer position is wrapped, and adjust the
data size calculation accordingly.

Example

  Use a very small buffer size (8K) and observe the size of truncated [T]
  data. Before the fix, it is possible to see records of 0 size.

  Before:

    $ perf record -m,8K -e intel_pt// uname
    Linux
    [ perf record: Woken up 2 times to write data ]
    [ perf record: Captured and wrote 0.105 MB perf.data ]
    $ perf script -D --no-itrace | grep AUX | grep -F '[T]'
    Warning:
    AUX data lost 2 times out of 3!

    5 19462712368111 0x19710 [0x40]: PERF_RECORD_AUX offset: 0 size: 0 flags: 0x1 [T]
    5 19462712700046 0x19ba8 [0x40]: PERF_RECORD_AUX offset: 0x170 size: 0xe90 flags: 0x1 [T]

 After:

    $ perf record -m,8K -e intel_pt// uname
    Linux
    [ perf record: Woken up 3 times to write data ]
    [ perf record: Captured and wrote 0.040 MB perf.data ]
    $ perf script -D --no-itrace | grep AUX | grep -F '[T]'
    Warning:
    AUX data lost 2 times out of 3!

    1 113720802995 0x4948 [0x40]: PERF_RECORD_AUX offset: 0 size: 0x2000 flags: 0x1 [T]
    1 113720979812 0x6b10 [0x40]: PERF_RECORD_AUX offset: 0x2000 size: 0x2000 flags: 0x1 [T]

Fixes: 52ca9ced ("perf/x86/intel/pt: Add Intel PT PMU driver")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20241022155920.17511-2-adrian.hunter@intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>