btrace: read entire aux buffer

btrace: read entire aux buffer

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