[Qemu-devel] [PATCH v12 17/25] gdbstub: add reverse continue support in replay mode

[Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>]

This patch adds support of the reverse continue operation for gdbstub.
Reverse continue finds the last breakpoint that would happen in normal
execution from the beginning to the current moment.
Implementation of the reverse continue replays the execution twice:
to find the breakpoints that were hit and to seek to the last breakpoint.
Reverse continue loads the previous snapshot and tries to find the breakpoint
since that moment. If there are no such breakpoints, it proceeds to
the earlier snapshot, and so on. When no breakpoints or watchpoints were
hit at all, execution stops at the beginning of the replay log.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
---
 cpus.c                    |    5 +++
 exec.c                    |    1 +
 gdbstub.c                 |   10 ++++++
 include/sysemu/replay.h   |    8 +++++
 replay/replay-debugging.c |   71 +++++++++++++++++++++++++++++++++++++++++++++
 stubs/replay.c            |    5 +++
 6 files changed, 99 insertions(+), 1 deletion(-)

diff --git a/cpus.c b/cpus.c
index 013341ca4f..6ef0992971 100644
--- a/cpus.c
+++ b/cpus.c
@@ -1110,6 +1110,11 @@ static void cpu_handle_guest_debug(CPUState *cpu)
         cpu->stopped = true;
     } else {
         if (!cpu->singlestep_enabled) {
+            /*
+             * Report about the breakpoint and
+             * make a single step to skip it
+             */
+            replay_breakpoint();
             cpu_single_step(cpu, SSTEP_ENABLE);
         } else {
             cpu_single_step(cpu, 0);
diff --git a/exec.c b/exec.c
index fdbd65c5b8..66fe4c36d9 100644
--- a/exec.c
+++ b/exec.c
@@ -2747,6 +2747,7 @@ static void check_watchpoint(int offset, int len, MemTxAttrs attrs, int flags)
                  * Don't process the watchpoints when we are
                  * in a reverse debugging operation.
                  */
+                replay_breakpoint();
                 return;
             }
             if (flags == BP_MEM_READ) {
diff --git a/gdbstub.c b/gdbstub.c
index d9bf3863c1..3d10f75a37 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -1439,6 +1439,14 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
                     put_packet(s, "E14");
                     break;
                 }
+            case 'c':
+                if (replay_reverse_continue()) {
+                    gdb_continue(s);
+                    return RS_IDLE;
+                } else {
+                    put_packet(s, "E14");
+                    break;
+                }
             default:
                 goto unknown_command;
             }
@@ -1749,7 +1757,7 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
             pstrcat(buf, sizeof(buf), ";multiprocess+");
 
             if (replay_mode == REPLAY_MODE_PLAY) {
-                pstrcat(buf, sizeof(buf), ";ReverseStep+");
+                pstrcat(buf, sizeof(buf), ";ReverseStep+;ReverseContinue+");
             }
 
             put_packet(s, buf);
diff --git a/include/sysemu/replay.h b/include/sysemu/replay.h
index 533003f2b0..1d18c9b6ea 100644
--- a/include/sysemu/replay.h
+++ b/include/sysemu/replay.h
@@ -79,11 +79,19 @@ const char *replay_get_filename(void);
  * Returns true on success.
  */
 bool replay_reverse_step(void);
+/*
+ * Start searching the last breakpoint/watchpoint.
+ * Used by gdbstub for backwards debugging.
+ * Returns true if the process successfully started.
+ */
+bool replay_reverse_continue(void);
 /*
  * Returns true if replay module is processing
  * reverse_continue or reverse_step request
  */
 bool replay_running_debug(void);
+/* Called in reverse debugging mode to collect breakpoint information */
+void replay_breakpoint(void);
 
 /* Processing the instructions */
 
diff --git a/replay/replay-debugging.c b/replay/replay-debugging.c
index 0c7e34f50f..9bbab5f02b 100644
--- a/replay/replay-debugging.c
+++ b/replay/replay-debugging.c
@@ -22,6 +22,8 @@
 #include "migration/snapshot.h"
 
 static bool replay_is_debugging;
+static int64_t replay_last_breakpoint;
+static int64_t replay_last_snapshot;
 
 bool replay_running_debug(void)
 {
@@ -252,3 +254,72 @@ bool replay_reverse_step(void)
 
     return false;
 }
+
+static void replay_continue_end(void)
+{
+    replay_is_debugging = false;
+    vm_stop(RUN_STATE_DEBUG);
+    replay_break(-1LL, NULL, NULL);
+}
+
+static void replay_continue_stop(void *opaque)
+{
+    Error *err = NULL;
+    if (replay_last_breakpoint != -1LL) {
+        replay_seek(replay_last_breakpoint, replay_stop_vm_debug, &err);
+        if (err) {
+            error_free(err);
+            replay_continue_end();
+        }
+        return;
+    }
+    /*
+     * No breakpoints since the last snapshot.
+     * Find previous snapshot and try again.
+     */
+    if (replay_last_snapshot != 0) {
+        replay_seek(replay_last_snapshot - 1, replay_continue_stop, &err);
+        if (err) {
+            error_free(err);
+            replay_continue_end();
+        }
+        replay_last_snapshot = replay_get_current_step();
+        return;
+    } else {
+        /* Seek to the very first step */
+        replay_seek(0, replay_stop_vm_debug, &err);
+        if (err) {
+            error_free(err);
+            replay_continue_end();
+        }
+        return;
+    }
+    replay_continue_end();
+}
+
+bool replay_reverse_continue(void)
+{
+    Error *err = NULL;
+
+    assert(replay_mode == REPLAY_MODE_PLAY);
+
+    if (replay_get_current_step() != 0) {
+        replay_seek(replay_get_current_step() - 1, replay_continue_stop, &err);
+        if (err) {
+            error_free(err);
+            return false;
+        }
+        replay_last_breakpoint = -1LL;
+        replay_is_debugging = true;
+        replay_last_snapshot = replay_get_current_step();
+        return true;
+    }
+
+    return false;
+}
+
+void replay_breakpoint(void)
+{
+    assert(replay_mode == REPLAY_MODE_PLAY);
+    replay_last_breakpoint = replay_get_current_step();
+}
diff --git a/stubs/replay.c b/stubs/replay.c
index 521552fa83..ee649726f4 100644
--- a/stubs/replay.c
+++ b/stubs/replay.c
@@ -85,3 +85,8 @@ bool replay_reverse_step(void)
 {
     return false;
 }
+
+bool replay_reverse_continue(void)
+{
+    return false;
+}