[PATCH 3/3] gdb/amd-dbgapi: add displaced stepping support

[simon.marchi@polymtl.ca]

From: Simon Marchi <simon.marchi@efficios.com>

Implement the target_ops displaced stepping methods to add displaced
stepping support when debugging AMD GPU programs.  The knowledge of how
to prepare and finish displaced steps is provided by the amd-dbgapi
library, so the code here is relatively straightforward.  No need to
parse instructions or handle fixups, that is done by the lib  We just
need to remember, for each thread doing a displaced step, the displaced
stepping id given by the library.

Add a test to exercise the new functionality.  The compiler generates
DWARF that GDB doesn't understand yet [1], so trying to step over a
breakpoint with DWARF present gives:

    (gdb) si
    Unhandled dwarf expression opcode 0xe9

The test purposefully builds the binary without DWARF info to circumvent
this.

[1] https://llvm.org/docs/AMDGPUDwarfExtensionsForHeterogeneousDebugging.html

Change-Id: I53f459221a42d4b02a6041eadb8cf554500e2162
---
 gdb/amd-dbgapi-target.c                       | 138 ++++++++++++++++++
 gdb/testsuite/gdb.rocm/displaced-stepping.exp |  54 +++++++
 2 files changed, 192 insertions(+)
 create mode 100644 gdb/testsuite/gdb.rocm/displaced-stepping.exp

diff --git a/gdb/amd-dbgapi-target.c b/gdb/amd-dbgapi-target.c
index 153a35f788ed..996e16abf591 100644
--- a/gdb/amd-dbgapi-target.c
+++ b/gdb/amd-dbgapi-target.c
@@ -24,6 +24,7 @@
 #include "cli/cli-cmds.h"
 #include "cli/cli-decode.h"
 #include "cli/cli-style.h"
+#include "gdbcore.h"
 #include "gdbsupport/unordered_map.h"
 #include "inf-loop.h"
 #include "inferior.h"
@@ -215,6 +216,12 @@ struct amd_dbgapi_inferior_info
   /* List of pending events the amd-dbgapi target retrieved from the dbgapi.  */
   std::list<std::pair<ptid_t, target_waitstatus>> wave_events;
 
+  /* Map of threads with ongoing displaced steps to corresponding amd-dbgapi
+     displaced stepping handles.  */
+  gdb::unordered_map<thread_info *,
+		     decltype (amd_dbgapi_displaced_stepping_id_t::handle)>
+    stepping_id_map;
+
   /* Map of wave ID to wave_info.  We cache wave_info objects because
      we need to access the info after the wave is gone, in the thread
      exit nofication.  E.g.:
@@ -291,6 +298,21 @@ struct amd_dbgapi_target final : public target_ops
   bool stopped_by_sw_breakpoint () override;
   bool stopped_by_hw_breakpoint () override;
 
+  bool supports_displaced_step (thread_info *thread) override
+  {
+    /* Handle displaced stepping for GPU threads only.  */
+    if (!ptid_is_gpu (thread->ptid))
+      return beneath ()->supports_displaced_step (thread);
+
+    return true;
+  }
+
+  displaced_step_prepare_status displaced_step_prepare
+    (thread_info *thread, CORE_ADDR &displaced_pc) override;
+
+  displaced_step_finish_status displaced_step_finish
+    (thread_info *thread, const target_waitstatus &status) override;
+
 private:
   /* True if we must report thread events.  */
   bool m_report_thread_events = false;
@@ -1897,6 +1919,122 @@ amd_dbgapi_target::update_thread_list ()
   this->beneath ()->update_thread_list ();
 }
 
+displaced_step_prepare_status
+amd_dbgapi_target::displaced_step_prepare (thread_info *thread,
+					   CORE_ADDR &displaced_pc)
+{
+  if (!ptid_is_gpu (thread->ptid))
+    return beneath ()->displaced_step_prepare (thread, displaced_pc);
+
+  gdb_assert (!thread->displaced_step_state.in_progress ());
+
+  /* Read the bytes that were overwritten by the breakpoint instruction being
+     stepped over.  */
+  CORE_ADDR original_pc = regcache_read_pc (get_thread_regcache (thread));
+  gdbarch *arch = get_thread_regcache (thread)->arch ();
+  size_t size = get_amdgpu_gdbarch_tdep (arch)->breakpoint_instruction_size;
+  gdb::byte_vector overwritten_bytes (size);
+
+  read_memory (original_pc, overwritten_bytes.data (), size);
+
+  /* Ask dbgapi to start the displaced step.  */
+  amd_dbgapi_wave_id_t wave_id = get_amd_dbgapi_wave_id (thread->ptid);
+  amd_dbgapi_displaced_stepping_id_t stepping_id;
+  amd_dbgapi_status_t status
+    = amd_dbgapi_displaced_stepping_start (wave_id, overwritten_bytes.data (),
+					   &stepping_id);
+
+  switch (status)
+   {
+    case AMD_DBGAPI_STATUS_SUCCESS:
+      break;
+
+    case AMD_DBGAPI_STATUS_ERROR_DISPLACED_STEPPING_BUFFER_NOT_AVAILABLE:
+      return DISPLACED_STEP_PREPARE_STATUS_UNAVAILABLE;
+
+    case AMD_DBGAPI_STATUS_ERROR_ILLEGAL_INSTRUCTION:
+      return DISPLACED_STEP_PREPARE_STATUS_CANT;
+
+    default:
+      error (_("amd_dbgapi_displaced_stepping_start failed (%s)"),
+	   get_status_string (status));
+    }
+
+  /* Save the displaced stepping id in the per-inferior info.  */
+  amd_dbgapi_inferior_info *info = get_amd_dbgapi_inferior_info (thread->inf);
+
+  bool inserted
+    = info->stepping_id_map.emplace (thread, stepping_id.handle).second;
+  gdb_assert (inserted);
+
+  /* Get the new (displaced) PC.  */
+  status = amd_dbgapi_wave_get_info (wave_id, AMD_DBGAPI_WAVE_INFO_PC,
+				     sizeof (displaced_pc), &displaced_pc);
+  if (status != AMD_DBGAPI_STATUS_SUCCESS)
+    {
+      amd_dbgapi_displaced_stepping_complete (wave_id, stepping_id);
+      error (_("amd_dbgapi_wave_get_info failed (%s), could not get the "
+	       "thread's displaced PC."),
+	     get_status_string (status));
+    }
+
+  displaced_debug_printf ("selected buffer at %#lx", displaced_pc);
+
+  /* We may have written some registers, so flush the register cache.  */
+  registers_changed_thread (thread);
+
+  return DISPLACED_STEP_PREPARE_STATUS_OK;
+}
+
+displaced_step_finish_status
+amd_dbgapi_target::displaced_step_finish (thread_info *thread,
+					  const target_waitstatus &ws)
+{
+  if (!ptid_is_gpu (thread->ptid))
+    return beneath ()->displaced_step_finish (thread, ws);
+
+  gdb_assert (thread->displaced_step_state.in_progress ());
+
+  /* Find the displaced stepping id for this thread.  */
+  amd_dbgapi_inferior_info *info = get_amd_dbgapi_inferior_info (thread->inf);
+  auto entry = info->stepping_id_map.extract (thread);
+
+  gdb_assert (entry.has_value ());
+  amd_dbgapi_displaced_stepping_id_t stepping_id {entry->second};
+
+  /* If the thread exited while stepping, we are done.  The code above
+     cleared our associated resources.  We don't want to call dbgapi
+     below: since the thread is gone, we wouldn't be able to find the
+     necessary wave ID.  dbgapi already took care of releasing its
+     displaced-stepping-related resources when it deleted the
+     wave.  */
+  if (ws.kind () == TARGET_WAITKIND_THREAD_EXITED)
+    return DISPLACED_STEP_FINISH_STATUS_OK;
+
+  amd_dbgapi_wave_id_t wave_id = get_amd_dbgapi_wave_id (thread->ptid);
+  amd_dbgapi_wave_stop_reasons_t stop_reason;
+  amd_dbgapi_status_t status
+    = amd_dbgapi_wave_get_info (wave_id, AMD_DBGAPI_WAVE_INFO_STOP_REASON,
+				sizeof (stop_reason), &stop_reason);
+
+  if (status != AMD_DBGAPI_STATUS_SUCCESS)
+    error (_("wave_get_info for wave_%ld failed (%s)"), wave_id.handle,
+	   get_status_string (status));
+
+  status = amd_dbgapi_displaced_stepping_complete (wave_id, stepping_id);
+
+  if (status != AMD_DBGAPI_STATUS_SUCCESS)
+    error (_("amd_dbgapi_displaced_stepping_complete failed (%s)"),
+	   get_status_string (status));
+
+  /* We may have written some registers, so flush the register cache.  */
+  registers_changed_thread (thread);
+
+  return (stop_reason & AMD_DBGAPI_WAVE_STOP_REASON_SINGLE_STEP) != 0
+	   ? DISPLACED_STEP_FINISH_STATUS_OK
+	   : DISPLACED_STEP_FINISH_STATUS_NOT_EXECUTED;
+}
+
 /* inferior_created observer.  */
 
 static void
diff --git a/gdb/testsuite/gdb.rocm/displaced-stepping.exp b/gdb/testsuite/gdb.rocm/displaced-stepping.exp
new file mode 100644
index 000000000000..6b26e1adfd7a
--- /dev/null
+++ b/gdb/testsuite/gdb.rocm/displaced-stepping.exp
@@ -0,0 +1,54 @@
+# Copyright 2024 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test displaced stepping on AMD GPUs.
+
+load_lib rocm.exp
+
+standard_testfile simple.cpp
+
+require allow_hipcc_tests
+
+# Since GDB doesn't yet understand DWARF expressions generated by the HIP
+# compiler, purposefully generate the binary without debug info.
+if {[build_executable "failed to prepare" $testfile $srcfile {hip}]} {
+    return
+}
+
+proc do_test {} {
+    clean_restart $::binfile
+
+    with_rocm_gpu_lock {
+	if ![runto_main] {
+	    return
+	}
+
+	gdb_test "with breakpoint pending on -- break do_an_addition" \
+	    "Breakpoint $::decimal \\(do_an_addition\\) pending."
+
+	gdb_test "continue" \
+	    "Thread $::decimal hit Breakpoint $::decimal, $::hex in do_an_addition.*"
+
+	gdb_test_no_output "set debug displaced on"
+
+	gdb_test "stepi" "displaced_step_prepare_throw: prepared successfully.*$::hex in do_an_addition.*"
+
+	gdb_test "continue" \
+	    "Inferior 1 .* exited normally.*" \
+	    "continue to end"
+    }
+}
+
+do_test
-- 
2.47.1