remote+docs: software/hardware breakpoint traps

This adjusts target remote to tell the core whether a trap was caused
by a breakpoint.

To that end, the patch teaches GDB about new RSP stop reasons "T05
swbreak" and "T05 hwbreak", that remote targets report back to GDB,
similarly to how "T05 watch" indicates a stop caused by a watchpoint.

Because targets that can report these events are expected to
themselves adjust the PC after a software breakpoint, these new stop
reasons must only be reported if the stub is talking to a GDB that
understands them.  Because of that, the use of the new stop reasons
needs to be handshaked on initial connection, using the qSupported
mechanism.  GDB simply sends "swbreak+" in its qSupports query, and
the stub reports back "swbreak+" too.

Because these new stop reasons are required to fix a fundamental
non-stop mode problem, this commit extends the remote non-stop intro
section in the manual, documenting the events as required.

To be clear, GDB will still cope with remote targets that don't
support these new stop reasons; it will behave just like today.

Tested on x86-64 Fedora 20, native and gdbserver.

gdb/ChangeLog:
2015-03-04  Pedro Alves  <palves@redhat.com>

	* NEWS: Mention the new "swbreak" and "hwbreak" stop reasons.
	* remote.c (struct remote_state) <remote_stopped_by_watchpoint_p>:
	Delete field.
	<stop_reason>: New field.
	(PACKET_swbreak_feature, PACKET_hwbreak_feature): New enum values.
	(packet_set_cmd_state): New function.
	(remote_protocol_features): Register the "swbreak" and "hwbreak"
	features.
	(remote_query_supported): If not disabled with the corresponding
	"set remote foo-packet" command, report support for the swbreak
	and hwbreak features.
	(struct stop_reply) <remote_stopped_by_watchpoint_p>: Delete
	field.
	<stop_reason>: New field.
	(remote_parse_stop_reply): Handle "swbreak" and "hwbreak".
	(remote_wait_as): Adjust.
	(remote_stopped_by_sw_breakpoint)
	(remote_supports_stopped_by_sw_breakpoint)
	(remote_stopped_by_hw_breakpoint)
	(remote_supports_stopped_by_hw_breakpoint): New functions.
	(remote_stopped_by_watchpoint): New function.
	(init_remote_ops): Install them.
	(_initialize_remote): Register new "set/show remote
	swbreak-feature-packet" and "set/show remote
	swbreak-feature-packet" commands.

gdb/doc/ChangeLog:
2015-03-04  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Remote Configuration): Document the "set/show
	remote swbreak-feature-packet" and "set/show remote
	hwbreak-feature-packet" commands.
	(Packets) <Z0>: Add cross link to the "swbreak" stop reason's
	decription.
	(Stop Reply Packets): Document the swbreak and hwbreak stop
	reasons.
	(General Query Packets): Document the swbreak and hwbreak
	qSupported features.
	(Remote Non-Stop): Explain that swbreak and hwbreak are required.

2 files changed, 93 insertions, 0 deletions

diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index 2b2ad5a..f62b4ac6 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,16 @@
+2015-03-04  Pedro Alves  <palves@redhat.com>
+
+	* gdb.texinfo (Remote Configuration): Document the "set/show
+	remote swbreak-feature-packet" and "set/show remote
+	hwbreak-feature-packet" commands.
+	(Packets) <Z0>: Add cross link to the "swbreak" stop reason's
+	decription.
+	(Stop Reply Packets): Document the swbreak and hwbreak stop
+	reasons.
+	(General Query Packets): Document the swbreak and hwbreak
+	qSupported features.
+	(Remote Non-Stop): Explain that swbreak and hwbreak are required.
+
 2015-03-03  Philippe Proulx  eeppeliteloop@gmail.com
 
 	* gdb.texinfo (gdb/mi Async Records): Fix duplicate syscall-entry
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index c0a8244..4b76ce9 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -19783,6 +19783,15 @@ are:
 @item @code{conditional-breakpoints-packet}
 @tab @code{Z0 and Z1}
 @tab @code{Support for target-side breakpoint condition evaluation}
+
+@item @code{swbreak-feature}
+@tab @code{swbreak stop reason}
+@tab @code{break}
+
+@item @code{hwbreak-feature}
+@tab @code{hwbreak stop reason}
+@tab @code{hbreak}
+
 @end multitable
 
 @node Remote Stub
@@ -34985,6 +34994,9 @@ form that should be evaluated on the target's side.  These are the
 conditions that should be taken into consideration when deciding if
 the breakpoint trigger should be reported back to @var{GDBN}.
 
+See also the @samp{swbreak} stop reason (@pxref{swbreak stop reason})
+for how to best report a memory breakpoint event to @value{GDBN}.
+
 The @var{cond_list} parameter is comprised of a series of expressions,
 concatenated without separators. Each expression has the following form:
 
@@ -35188,6 +35200,34 @@ logged execution events, because it has reached the end (or the
 beginning when executing backward) of the log.  The value of @var{r}
 will be either @samp{begin} or @samp{end}.  @xref{Reverse Execution}, 
 for more information.
+
+@item swbreak
+@anchor{swbreak stop reason}
+The packet indicates a memory breakpoint instruction was executed,
+irrespective of whether it was @value{GDBN} that planted the
+breakpoint or the breakpoint is hardcoded in the program.  The @var{r}
+part must be left empty.
+
+On some architectures, such as x86, at the architecture level, when a
+breakpoint instruction executes the program counter points at the
+breakpoint address plus an offset.  On such targets, the stub is
+responsible for adjusting the PC to point back at the breakpoint
+address.
+
+This packet should not be sent by default; older @value{GDBN} versions
+did not support it.  @value{GDBN} requests it, by supplying an
+appropriate @samp{qSupported} feature (@pxref{qSupported}).  The
+remote stub must also supply the appropriate @samp{qSupported} feature
+indicating support.
+
+This packet is required for correct non-stop mode operation.
+
+@item hwbreak
+The packet indicates the target stopped for a hardware breakpoint.
+The @var{r} part must be left empty.
+
+The same remarks about @samp{qSupported} and non-stop mode above
+apply.
 @end table
 
 @item W @var{AA}
@@ -35772,6 +35812,14 @@ description.
 This feature indicates whether @value{GDBN} supports the
 @samp{qRelocInsn} packet (@pxref{Tracepoint Packets,,Relocate
 instruction reply packet}).
+
+@item swbreak
+This feature indicates whether @value{GDBN} supports the swbreak stop
+reason in stop replies.  @xref{swbreak stop reason}, for details.
+
+@item hwbreak
+This feature indicates whether @value{GDBN} supports the hwbreak stop
+reason in stop replies.  @xref{swbreak stop reason}, for details.
 @end table
 
 Stubs should ignore any unknown values for
@@ -36000,6 +36048,16 @@ These are the currently defined stub features and their properties:
 @tab @samp{-}
 @tab No
 
+@item @samp{swbreak}
+@tab No
+@tab @samp{-}
+@tab No
+
+@item @samp{hwbreak}
+@tab No
+@tab @samp{-}
+@tab No
+
 @end multitable
 
 These are the currently defined stub features, in more detail:
@@ -36180,6 +36238,14 @@ The remote stub understands the @samp{Qbtrace:bts} packet.
 @item Qbtrace-conf:bts:size
 The remote stub understands the @samp{Qbtrace-conf:bts:size} packet.
 
+@item swbreak
+The remote stub reports the @samp{swbreak} stop reason for memory
+breakpoints.
+
+@item hwbreak
+The remote stub reports the @samp{hwbreak} stop reason for hardware
+breakpoints.
+
 @end table
 
 @item qSymbol::
@@ -37565,6 +37631,20 @@ If all threads are running when the target receives the @samp{?} packet,
 or if the target is not attached to any process, it shall respond
 @samp{OK}.
 
+If the stub supports non-stop mode, it should also support the
+@samp{swbreak} stop reason if software breakpoints are supported, and
+the @samp{hwbreak} stop reason if hardware breakpoints are supported
+(@pxref{swbreak stop reason}).  This is because given the asynchronous
+nature of non-stop mode, between the time a thread hits a breakpoint
+and the time the event is finally processed by @value{GDBN}, the
+breakpoint may have already been removed from the target.  Due to
+this, @value{GDBN} needs to be able to tell whether a trap stop was
+caused by a delayed breakpoint event, which should be ignored, as
+opposed to a random trap signal, which should be reported to the user.
+Note the @samp{swbreak} feature implies that the target is responsible
+for adjusting the PC when a software breakpoint triggers, if
+necessary, such as on the x86 architecture.
+
 @node Packet Acknowledgment
 @section Packet Acknowledgment