[PATCH 5/7] Disable Z0 packet on aarch64 on multi-arch debugging

[Yao Qi <qiyaoltc at gmail dot com>]

In multi-arch debugging, if GDB sends Z0 packet, GDBserver should be
able to do several things below:

 - choose the right breakpoint instruction to insert according to the
   information available, such as 'kind' in Z0 packet and address,

 - choose the right breakpoint instruction to check memory writes and
   validate inserted memory breakpoint

 - be aware of different breakpoint instructions in $ARCH_breakpoint_at.

unfortunately GDBserver can't do them now.  Although x86 GDBserver
supports multi-arch, it doesn't need to support them above because
breakpoint instruction on i686 and x86_64 is the same.  However,
breakpoint instructions on aarch64 and arm (arm mode, thumb1, and thumb2)
are different.

I tried to teach aarch64 GDBserver backend to be really
multi-arch-capable in the following ways,

 - linux_low_target return the right breakpoint instruction according to
   the 'kind' in Z0 packet, and insert_memory_breakpoint can do the right
   thing.
 - once breakpoint is inserted, the breakpoint data and length is recorded
   in each breakpoint object, so that validate_breakpoint and
   check_mem_write can get the right breakpoint instruction from each
   breakpoint object, rather than from global variable breakpoint_data.
 - linux_low_target needs another hook function for pc increment after
   hitting a breakpoint.
 - let set_breakpoint_at, which is widely used for tracepoint, use the
   'default' breakpoint instruction.  We can always use aarch64 breakpoint
   instruction since arm doesn't support tracepoint yet.

looks it is not a small piece of work, so I decide to disable Z0 packet
on multi-arch, which means aarch64 GDBserver only supports Z0 packet
if it is started to debug only one process (extended protocol is not
used) and process target description is 64-bit.

gdb/gdbserver:

2015-07-28  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch64-low.c (aarch64_supports_z_point_type): Return
	0 for Z_PACKET_SW_BP if it may be used in multi-arch debugging.
	* server.c (extended_protocol): Remove "static".
	* server.h (extended_protocol): Declare it.
---
 gdb/gdbserver/linux-aarch64-low.c | 16 ++++++++++++++++
 gdb/gdbserver/server.c            |  2 +-
 gdb/gdbserver/server.h            |  1 +
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/gdb/gdbserver/linux-aarch64-low.c b/gdb/gdbserver/linux-aarch64-low.c
index 3512ce9..90d2b43 100644
--- a/gdb/gdbserver/linux-aarch64-low.c
+++ b/gdb/gdbserver/linux-aarch64-low.c
@@ -364,6 +364,22 @@ aarch64_supports_z_point_type (char z_type)
   switch (z_type)
     {
     case Z_PACKET_SW_BP:
+      {
+	if (!extended_protocol && is_64bit_tdesc ())
+	  {
+	    /* Only enable Z0 packet in non-multi-arch debugging.  If
+	       extended protocol is used, don't enable Z0 packet because
+	       GDBserver may attach to 32-bit process.  */
+	    return 1;
+	  }
+	else
+	  {
+	    /* Disable Z0 packet so that GDBserver doesn't have to handle
+	       different breakpoint instructions (aarch64, arm, thumb etc)
+	       in multi-arch debugging.  */
+	    return 0;
+	  }
+      }
     case Z_PACKET_HW_BP:
     case Z_PACKET_WRITE_WP:
     case Z_PACKET_READ_WP:
diff --git a/gdb/gdbserver/server.c b/gdb/gdbserver/server.c
index 2918770..f15b7be 100644
--- a/gdb/gdbserver/server.c
+++ b/gdb/gdbserver/server.c
@@ -49,7 +49,7 @@ ptid_t general_thread;
 
 int server_waiting;
 
-static int extended_protocol;
+int extended_protocol;
 static int response_needed;
 static int exit_requested;
 
diff --git a/gdb/gdbserver/server.h b/gdb/gdbserver/server.h
index 09a5624..9080151 100644
--- a/gdb/gdbserver/server.h
+++ b/gdb/gdbserver/server.h
@@ -87,6 +87,7 @@ extern int multi_process;
 extern int report_fork_events;
 extern int report_vfork_events;
 extern int non_stop;
+extern int extended_protocol;
 
 /* True if the "swbreak+" feature is active.  In that case, GDB wants
    us to report whether a trap is explained by a software breakpoint
-- 
1.9.1