The data_head of a perf event data buffer grows indefinitely. Users are
expected to compute data_head % data_size to find the location inside the perf
event data buffer.
The aux_head of a perf event aux buffer wraps around and always stays within the
perf event aux buffer.
Well, at least that's the behaviour for BTS and PT - where BTS uses the data
buffer and PT the aux buffer.
GDB does not read beyond data_head or aux_head. This is OK for BTS but wrong
for PT. It causes only a portion of the trace to be considered by GDB. In the
extreme case, the buffer may appear (almost) empty.
Thanks to Tim Wiederhake <tim.wiederhake@intel.com> for reporting the anomaly.
Change it to read the entire aux buffer for PT. The buffer is initially zero so
any extra zeroes we read before aux_head wraps around the first time will be
ignored when searching for the first PSB packet in order to synchronize onto the
trace stream.
gdb/
* nat/linux-btrace.c (perf_event_read): Allow data_head < size.
* nat/linux-btrace.c (perf_event_read_all): Do not adjust size.
Change-Id: If4f8049a2080a5f16f336309450b32a3eb1e3ec9
db58b3735f