gdbstub: add reverse step support in replay mode

GDB remote protocol supports two reverse debugging commands:
reverse step and reverse continue.
This patch adds support of the first one to the gdbstub.
Reverse step is intended to step one instruction in the backwards
direction. This is not possible in regular execution.
But replayed execution is deterministic, therefore we can load one of
the prior snapshots and proceed to the desired step. It is equivalent
to stepping one instruction back.
There should be at least one snapshot preceding the debugged part of
the replay log.

Signed-off-by: Pavel Dovgalyuk <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
--

v4 changes:
 - inverted condition in cpu_handle_guest_debug (suggested by Alex Bennée)
Message-Id: <160174522341.12451.1498758422543765253.stgit@pasha-ThinkPad-X280>

Signed-off-by: Paolo Bonzini <[email protected]>

diff --git a/accel/tcg/translator.c b/accel/tcg/translator.c
index 603d17ff831df0e4a9b8446666e91139605a8597..fb1e19c58585236788e3c7f15f8c0a409ace458f 100644 (file)
--- a/accel/tcg/translator.c
+++ b/accel/tcg/translator.c
@@ -17,6 +17,7 @@
 #include "exec/log.h"
 #include "exec/translator.h"
 #include "exec/plugin-gen.h"
+#include "sysemu/replay.h"
 
 /* Pairs with tcg_clear_temp_count.
    To be called by #TranslatorOps.{translate_insn,tb_stop} if

diff --git a/exec.c b/exec.c
index 2305f5553e45cd6b9d4d37b6a9f7ffb1e74eda72..d8072668eaca326e1c37784b9df0f0758e05df3a 100644 (file)
--- a/exec.c
+++ b/exec.c
@@ -2748,6 +2748,13 @@ void cpu_check_watchpoint(CPUState *cpu, vaddr addr, vaddr len,
     QTAILQ_FOREACH(wp, &cpu->watchpoints, entry) {
         if (watchpoint_address_matches(wp, addr, len)
             && (wp->flags & flags)) {
+            if (replay_running_debug()) {
+                /*
+                 * Don't process the watchpoints when we are
+                 * in a reverse debugging operation.
+                 */
+                return;
+            }
             if (flags == BP_MEM_READ) {
                 wp->flags |= BP_WATCHPOINT_HIT_READ;
             } else {

diff --git a/gdbstub.c b/gdbstub.c
index 9dfb6e4142cb5b8f2c44ae79c9758b3b568d05a2..79e8ccc0503cc8d14a3d7a1fef4e71b8e8fd256f 100644 (file)
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -51,6 +51,7 @@
 #include "sysemu/runstate.h"
 #include "hw/semihosting/semihost.h"
 #include "exec/exec-all.h"
+#include "sysemu/replay.h"
 
 #ifdef CONFIG_USER_ONLY
 #define GDB_ATTACHED "0"
@@ -375,6 +376,20 @@ typedef struct GDBState {
  */
 static int sstep_flags = SSTEP_ENABLE|SSTEP_NOIRQ|SSTEP_NOTIMER;
 
+/* Retrieves flags for single step mode. */
+static int get_sstep_flags(void)
+{
+    /*
+     * In replay mode all events written into the log should be replayed.
+     * That is why NOIRQ flag is removed in this mode.
+     */
+    if (replay_mode != REPLAY_MODE_NONE) {
+        return SSTEP_ENABLE;
+    } else {
+        return sstep_flags;
+    }
+}
+
 static GDBState gdbserver_state;
 
 static void init_gdbserver_state(void)
@@ -501,7 +516,7 @@ static int gdb_continue_partial(char *newstates)
                 break; /* nothing to do here */
             case 's':
                 trace_gdbstub_op_stepping(cpu->cpu_index);
-                cpu_single_step(cpu, sstep_flags);
+                cpu_single_step(cpu, get_sstep_flags());
                 cpu_resume(cpu);
                 flag = 1;
                 break;
@@ -1874,10 +1889,31 @@ static void handle_step(GdbCmdContext *gdb_ctx, void *user_ctx)
         gdb_set_cpu_pc((target_ulong)gdb_ctx->params[0].val_ull);
     }
 
-    cpu_single_step(gdbserver_state.c_cpu, sstep_flags);
+    cpu_single_step(gdbserver_state.c_cpu, get_sstep_flags());
     gdb_continue();
 }
 
+static void handle_backward(GdbCmdContext *gdb_ctx, void *user_ctx)
+{
+    if (replay_mode != REPLAY_MODE_PLAY) {
+        put_packet("E22");
+    }
+    if (gdb_ctx->num_params == 1) {
+        switch (gdb_ctx->params[0].opcode) {
+        case 's':
+            if (replay_reverse_step()) {
+                gdb_continue();
+            } else {
+                put_packet("E14");
+            }
+            return;
+        }
+    }
+
+    /* Default invalid command */
+    put_packet("");
+}
+
 static void handle_v_cont_query(GdbCmdContext *gdb_ctx, void *user_ctx)
 {
     put_packet("vCont;c;C;s;S");
@@ -2124,6 +2160,10 @@ static void handle_query_supported(GdbCmdContext *gdb_ctx, void *user_ctx)
         g_string_append(gdbserver_state.str_buf, ";qXfer:features:read+");
     }
 
+    if (replay_mode == REPLAY_MODE_PLAY) {
+        g_string_append(gdbserver_state.str_buf, ";ReverseStep+");
+    }
+
     if (gdb_ctx->num_params &&
         strstr(gdb_ctx->params[0].data, "multiprocess+")) {
         gdbserver_state.multiprocess = true;
@@ -2460,6 +2500,17 @@ static int gdb_handle_packet(const char *line_buf)
             cmd_parser = &step_cmd_desc;
         }
         break;
+    case 'b':
+        {
+            static const GdbCmdParseEntry backward_cmd_desc = {
+                .handler = handle_backward,
+                .cmd = "b",
+                .cmd_startswith = 1,
+                .schema = "o0"
+            };
+            cmd_parser = &backward_cmd_desc;
+        }
+        break;
     case 'F':
         {
             static const GdbCmdParseEntry file_io_cmd_desc = {

diff --git a/include/sysemu/replay.h b/include/sysemu/replay.h
index c486c3afb2f166e2ec23cb330e97b67b1375b25c..90bbb5d825cc4e491eb1d37038411aa6dde5bcab 100644 (file)
--- a/include/sysemu/replay.h
+++ b/include/sysemu/replay.h
@@ -75,6 +75,17 @@ void replay_finish(void);
 void replay_add_blocker(Error *reason);
 /* Returns name of the replay log file */
 const char *replay_get_filename(void);
+/*
+ * Start making one step in backward direction.
+ * Used by gdbstub for backwards debugging.
+ * Returns true on success.
+ */
+bool replay_reverse_step(void);
+/*
+ * Returns true if replay module is processing
+ * reverse_continue or reverse_step request
+ */
+bool replay_running_debug(void);
 
 /* Processing the instructions */
 

diff --git a/replay/replay-debugging.c b/replay/replay-debugging.c
index e1fe6b8661338ead77a9c5fbc24cab7aa8b213f6..1e1dec0295bedd57e3d7c13a45370090dce3ab4c 100644 (file)
--- a/replay/replay-debugging.c
+++ b/replay/replay-debugging.c
@@ -22,6 +22,13 @@
 #include "block/snapshot.h"
 #include "migration/snapshot.h"
 
+static bool replay_is_debugging;
+
+bool replay_running_debug(void)
+{
+    return replay_is_debugging;
+}
+
 void hmp_info_replay(Monitor *mon, const QDict *qdict)
 {
     if (replay_mode == REPLAY_MODE_NONE) {
@@ -212,3 +219,30 @@ void hmp_replay_seek(Monitor *mon, const QDict *qdict)
         return;
     }
 }
+
+static void replay_stop_vm_debug(void *opaque)
+{
+    replay_is_debugging = false;
+    vm_stop(RUN_STATE_DEBUG);
+    replay_delete_break();
+}
+
+bool replay_reverse_step(void)
+{
+    Error *err = NULL;
+
+    assert(replay_mode == REPLAY_MODE_PLAY);
+
+    if (replay_get_current_icount() != 0) {
+        replay_seek(replay_get_current_icount() - 1,
+                    replay_stop_vm_debug, &err);
+        if (err) {
+            error_free(err);
+            return false;
+        }
+        replay_is_debugging = true;
+        return true;
+    }
+
+    return false;
+}

diff --git a/softmmu/cpus.c b/softmmu/cpus.c
index f3d0c59f785aa9ed4bdf1abb5f8208f058e6ed4d..95c557fac599741aac3bffca503bfd2977cda2ba 100644 (file)
--- a/softmmu/cpus.c
+++ b/softmmu/cpus.c
@@ -284,9 +284,17 @@ bool cpu_can_run(CPUState *cpu)
 
 void cpu_handle_guest_debug(CPUState *cpu)
 {
-    gdb_set_stop_cpu(cpu);
-    qemu_system_debug_request();
-    cpu->stopped = true;
+    if (replay_running_debug()) {
+        if (!cpu->singlestep_enabled) {
+            cpu_single_step(cpu, SSTEP_ENABLE);
+        } else {
+            cpu_single_step(cpu, 0);
+        }
+    } else {
+        gdb_set_stop_cpu(cpu);
+        qemu_system_debug_request();
+        cpu->stopped = true;
+    }
 }
 
 #ifdef CONFIG_LINUX

diff --git a/stubs/replay.c b/stubs/replay.c
index eacb366aa84eef0addbda18ac0944f520286f8b7..d5b52302e9713a5b37264c3af4f5e94ccd0379ce 100644 (file)
--- a/stubs/replay.c
+++ b/stubs/replay.c
@@ -93,3 +93,8 @@ uint64_t replay_get_current_icount(void)
 {
     return 0;
 }
+
+bool replay_reverse_step(void)
+{
+    return false;
+}