btrace, gdbserver: read branch trace incrementally

btrace, gdbserver: read branch trace incrementally

Read branch trace data incrementally and extend the current trace rather than
discarding it and reading the entire trace buffer each time.

If the branch trace buffer overflowed, we can't extend the current trace so we
discard it and start anew by reading the entire branch trace buffer.

2014-01-16  Markus Metzger  <markus.t.metzger@intel.com>

	* common/linux-btrace.c (perf_event_read_bts, linux_read_btrace):
	Support delta reads.
	(linux_disable_btrace): Change return type.
	* common/linux-btrace.h (linux_read_btrace): Change parameters
	and return type to allow error reporting.  Update users.
	(linux_disable_btrace): Change return type.  Update users.
	* common/btrace-common.h (btrace_read_type) <BTRACE_READ_DELTA>:
	New.
	(btrace_error): New.
	(btrace_block) <begin>: Comment on BEGIN == 0.
	* btrace.c (btrace_compute_ftrace): Start from the end of
	the current trace.
	(btrace_stitch_trace, btrace_clear_history): New.
	(btrace_fetch): Read delta trace, return if replaying.
	(btrace_clear): Move clear history code to btrace_clear_history.
	(parse_xml_btrace): Throw an error if parsing failed.
	* target.h (struct target_ops) <to_read_btrace>: Change parameters
	and return type to allow error reporting.
	(target_read_btrace): Change parameters and return type to allow
	error reporting.
	* target.c (target_read_btrace): Update.
	* remote.c (remote_read_btrace): Support delta reads.  Pass
	errors on.
	* NEWS: Announce it.

gdbserver/
	* target.h (target_ops) <read_btrace>: Change parameters and
	return type to allow error reporting.
	* server.c (handle_qxfer_btrace): Support delta reads.  Pass
	trace reading errors on.
	* linux-low.c (linux_low_read_btrace): Pass trace reading
	errors on.
	(linux_low_disable_btrace): New.