[Qemu-devel] [RFC PATCH v2 17/19] gdbstub: add reverse continue support in replay mode

[Pavel Dovgalyuk]

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 <address@hidden>
---
 cpus.c                    |    3 ++
 exec.c                    |    1 +
 gdbstub.c                 |   10 ++++++-
 include/sysemu/replay.h   |    6 ++++
 replay/replay-debugging.c |   69 +++++++++++++++++++++++++++++++++++++++++++++
 stubs/replay.c            |    5 +++
 6 files changed, 93 insertions(+), 1 deletion(-)

diff --git a/cpus.c b/cpus.c
index 3fb9321..9e81e9a 100644
--- a/cpus.c
+++ b/cpus.c
@@ -1048,6 +1048,9 @@ 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 0802aa4..e5fe4dd 100644
--- a/exec.c
+++ b/exec.c
@@ -2549,6 +2549,7 @@ static void check_watchpoint(int offset, int len, 
MemTxAttrs attrs, int flags)
             if (replay_running_debug()) {
                 /* 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 86dec56..1645b7f 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -1101,6 +1101,14 @@ 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;
             }
@@ -1371,7 +1379,7 @@ static int gdb_handle_packet(GDBState *s, const char 
*line_buf)
                 pstrcat(buf, sizeof(buf), ";qXfer:features:read+");
             }
             if (replay_mode == REPLAY_MODE_PLAY) {
-                pstrcat(buf, sizeof(buf), ";ReverseStep+");
+                pstrcat(buf, sizeof(buf), ";ReverseStep+;ReverseContinue+");
             }
             put_packet(s, buf);
             break;
diff --git a/include/sysemu/replay.h b/include/sysemu/replay.h
index 611eabb..a3113c1 100644
--- a/include/sysemu/replay.h
+++ b/include/sysemu/replay.h
@@ -78,9 +78,15 @@ void replay_break(int64_t step, QEMUTimerCB callback, void 
*opaque);
     Used by gdbstub for backwards debugging.
     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 388cf12..edab98e 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)
 {
@@ -216,3 +218,70 @@ 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, &err, replay_stop_vm_debug);
+        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, &err, replay_continue_stop);
+        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, &err, replay_stop_vm_debug);
+        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, &err, replay_continue_stop);
+        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 b0fba0e..781974e 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;
+}