[PATCH v4 11/15] gdbstub: add reverse continue support in replay mode

[Pavel Dovgalyuk <pavel.dovgalyuk@ispras.ru>]

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

diff --git a/exec.c b/exec.c
index 65983738f2..4333dde185 100644
--- a/exec.c
+++ b/exec.c
@@ -2756,6 +2756,7 @@ void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
                  * 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 79e8ccc050..ac92273018 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -1907,6 +1907,13 @@ static void handle_backward(GdbCmdContext *gdb_ctx, void *user_ctx)
                 put_packet("E14");
             }
             return;
+        case 'c':
+            if (replay_reverse_continue()) {
+                gdb_continue();
+            } else {
+                put_packet("E14");
+            }
+            return;
         }
     }
 
@@ -2161,7 +2168,8 @@ static void handle_query_supported(GdbCmdContext *gdb_ctx, void *user_ctx)
     }
 
     if (replay_mode == REPLAY_MODE_PLAY) {
-        g_string_append(gdbserver_state.str_buf, ";ReverseStep+");
+        g_string_append(gdbserver_state.str_buf,
+            ";ReverseStep+;ReverseContinue+");
     }
 
     if (gdb_ctx->num_params &&
diff --git a/include/sysemu/replay.h b/include/sysemu/replay.h
index 13a8123b09..b6cac175c4 100644
--- a/include/sysemu/replay.h
+++ b/include/sysemu/replay.h
@@ -81,11 +81,19 @@ 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 f7594a88cd..d02d4e0766 100644
--- a/replay/replay-debugging.c
+++ b/replay/replay-debugging.c
@@ -23,6 +23,8 @@
 #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)
 {
@@ -245,3 +247,72 @@ bool replay_reverse_step(void)
 
     return false;
 }
+
+static void replay_continue_end(void)
+{
+    replay_is_debugging = false;
+    vm_stop(RUN_STATE_DEBUG);
+    replay_delete_break();
+}
+
+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_icount();
+        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_icount() != 0) {
+        replay_seek(replay_get_current_icount() - 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_icount();
+        return true;
+    }
+
+    return false;
+}
+
+void replay_breakpoint(void)
+{
+    assert(replay_mode == REPLAY_MODE_PLAY);
+    replay_last_breakpoint = replay_get_current_icount();
+}
diff --git a/softmmu/cpus.c b/softmmu/cpus.c
index dc72f93bc0..756f268993 100644
--- a/softmmu/cpus.c
+++ b/softmmu/cpus.c
@@ -1006,6 +1006,11 @@ static void cpu_handle_guest_debug(CPUState *cpu)
 {
     if (replay_running_debug()) {
         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/stubs/replay.c b/stubs/replay.c
index d5b52302e9..45ebe77fb9 100644
--- a/stubs/replay.c
+++ b/stubs/replay.c
@@ -98,3 +98,8 @@ bool replay_reverse_step(void)
 {
     return false;
 }
+
+bool replay_reverse_continue(void)
+{
+    return false;
+}