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Re: [PATCH v4 20/24] replay: simple auto-snapshot mode for record

From: Pavel Dovgalyuk
Subject: Re: [PATCH v4 20/24] replay: simple auto-snapshot mode for record
Date: Tue, 12 Mar 2024 12:00:38 +0300
User-agent: Mozilla Thunderbird 
On 11.03.2024 20:40, Nicholas Piggin wrote:

record makes an initial snapshot when the machine is created, to enable
reverse-debugging. Often the issue being debugged appears near the end of
the trace, so it is important for performance to keep snapshots close to
the end.

This implements a periodic snapshot mode that keeps a rolling set of
recent snapshots. This could be done by the debugger or other program
that talks QMP, but for setting up simple scenarios and tests, this is
more convenient.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
  docs/system/replay.rst   |  5 ++++
  include/sysemu/replay.h  | 11 ++++++++
  replay/replay-snapshot.c | 57 ++++++++++++++++++++++++++++++++++++++++
  replay/replay.c          | 27 +++++++++++++++++--
  system/vl.c              |  9 +++++++
  qemu-options.hx          |  9 +++++--
  6 files changed, 114 insertions(+), 4 deletions(-)

diff --git a/docs/system/replay.rst b/docs/system/replay.rst
index ca7c17c63d..1ae8614475 100644
--- a/docs/system/replay.rst
+++ b/docs/system/replay.rst
@@ -156,6 +156,11 @@ for storing VM snapshots. Here is the example of the 
command line for this:
  ``empty.qcow2`` drive does not connected to any virtual block device and used
  for VM snapshots only.
+``rrsnapmode`` can be used to select just an initial snapshot or periodic 
+snapshots, with ``rrsnapcount`` specifying the number of periodic snapshots
+to maintain, and ``rrsnaptime`` the amount of run time in seconds between
+periodic snapshots.
+
  .. _network-label:
Network devices 
diff --git a/include/sysemu/replay.h b/include/sysemu/replay.h
index 8102fa54f0..92fa82842b 100644
--- a/include/sysemu/replay.h
+++ b/include/sysemu/replay.h
@@ -48,6 +48,17 @@ typedef enum ReplayCheckpoint ReplayCheckpoint;
typedef struct ReplayNetState ReplayNetState; +enum ReplaySnapshotMode { 
+    REPLAY_SNAPSHOT_MODE_INITIAL,
+    REPLAY_SNAPSHOT_MODE_PERIODIC,
+};
This should be defined in replay-internal.h, because it is internal for replay. 


+typedef enum ReplaySnapshotMode ReplaySnapshotMode;
+
+extern ReplaySnapshotMode replay_snapshot_mode;
+
+extern uint64_t replay_snapshot_periodic_delay;
+extern int replay_snapshot_periodic_nr_keep;


These ones are internal too.


+
  /* Name of the initial VM snapshot */
  extern char *replay_snapshot;
diff --git a/replay/replay-snapshot.c b/replay/replay-snapshot.c 
index ccb4d89dda..762555feaa 100644
--- a/replay/replay-snapshot.c
+++ b/replay/replay-snapshot.c
@@ -70,6 +70,53 @@ void replay_vmstate_register(void)
      vmstate_register(NULL, 0, &vmstate_replay, &replay_state);
  }
+static QEMUTimer *replay_snapshot_timer; 
+static int replay_snapshot_count;
+
+static void replay_snapshot_timer_cb(void *opaque)
+{
+    Error *err = NULL;
+    char *name;
+
+    if (!replay_can_snapshot()) {
+        /* Try again soon */
+        timer_mod(replay_snapshot_timer,
+                  qemu_clock_get_ms(QEMU_CLOCK_REALTIME) +
+                  replay_snapshot_periodic_delay / 10);
+        return;
+    }
+
+    name = g_strdup_printf("%s-%d", replay_snapshot, replay_snapshot_count);
+    if (!save_snapshot(name,
+                       true, NULL, false, NULL, &err)) {
+        error_report_err(err);
+        error_report("Could not create periodic snapshot "
+                     "for icount record, disabling");
+        g_free(name);
+        return;
+    }
+    g_free(name);
+    replay_snapshot_count++;
+
+    if (replay_snapshot_periodic_nr_keep >= 1 &&
+        replay_snapshot_count > replay_snapshot_periodic_nr_keep) {
+        int del_nr;
+
+        del_nr = replay_snapshot_count - replay_snapshot_periodic_nr_keep - 1;
+        name = g_strdup_printf("%s-%d", replay_snapshot, del_nr);


Copy-paste of snapshot name format.


+        if (!delete_snapshot(name, false, NULL, &err)) {
+            error_report_err(err);
+            error_report("Could not delete periodic snapshot "
+                         "for icount record");
+        }
+        g_free(name);
+    }
+
+    timer_mod(replay_snapshot_timer,
+              qemu_clock_get_ms(QEMU_CLOCK_REALTIME) +
+              replay_snapshot_periodic_delay);
+}
+
  void replay_vmstate_init(void)
  {
      Error *err = NULL;
@@ -82,6 +129,16 @@ void replay_vmstate_init(void)
                  error_report("Could not create snapshot for icount record");
                  exit(1);
              }
+
+            if (replay_snapshot_mode == REPLAY_SNAPSHOT_MODE_PERIODIC) {
+                replay_snapshot_timer = timer_new_ms(QEMU_CLOCK_REALTIME,
+                                                     replay_snapshot_timer_cb,
+                                                     NULL);
+                timer_mod(replay_snapshot_timer,
+                          qemu_clock_get_ms(QEMU_CLOCK_REALTIME) +
+                          replay_snapshot_periodic_delay);
+            }
+
          } else if (replay_mode == REPLAY_MODE_PLAY) {
              if (!load_snapshot(replay_snapshot, NULL, false, NULL, &err)) {
                  error_report_err(err);
diff --git a/replay/replay.c b/replay/replay.c
index 895fa6b67a..c916e71d30 100644
--- a/replay/replay.c
+++ b/replay/replay.c
@@ -29,6 +29,10 @@
  ReplayMode replay_mode = REPLAY_MODE_NONE;
  char *replay_snapshot;
+ReplaySnapshotMode replay_snapshot_mode; 
+uint64_t replay_snapshot_periodic_delay;
+int replay_snapshot_periodic_nr_keep;
+
  /* Name of replay file  */
  static char *replay_filename;
  ReplayState replay_state;
@@ -424,6 +428,27 @@ void replay_configure(QemuOpts *opts)
      }
replay_snapshot = g_strdup(qemu_opt_get(opts, "rrsnapshot")); 
+    if (replay_snapshot && mode == REPLAY_MODE_RECORD) {
+        const char *snapmode;
+
+        snapmode = qemu_opt_get(opts, "rrsnapmode");
+        if (!snapmode || !strcmp(snapmode, "initial")) {
+            replay_snapshot_mode = REPLAY_SNAPSHOT_MODE_INITIAL;
+        } else if (!strcmp(snapmode, "periodic")) {
+            replay_snapshot_mode = REPLAY_SNAPSHOT_MODE_PERIODIC;
+        } else {
+            error_report("Invalid rrsnapmode option: %s", snapmode);
+            exit(1);
+        }
+
+        /* Default 10 host seconds of machine runtime per snapshot. */
+        replay_snapshot_periodic_delay =
+                           qemu_opt_get_number(opts, "rrsnaptime", 10) * 1000;


Can user set it to zero here?


+
+        /* Default 2, to cover at least the last 10 host seconds of runtime. */
+        replay_snapshot_periodic_nr_keep =
+                           qemu_opt_get_number(opts, "rrsnapcount", 2);
+    }
      replay_vmstate_register();
      replay_enable(fname, mode);
@@ -446,8 +471,6 @@ void replay_start(void) 
          exit(1);
      }
- /* Timer for snapshotting will be set up here. */ 
-
      replay_enable_events();
  }
diff --git a/system/vl.c b/system/vl.c 
index 70f4cece7f..8070cee6da 100644
--- a/system/vl.c
+++ b/system/vl.c
@@ -447,6 +447,15 @@ static QemuOptsList qemu_icount_opts = {
          }, {
              .name = "rrsnapshot",
              .type = QEMU_OPT_STRING,
+        }, {
+            .name = "rrsnapmode",
+            .type = QEMU_OPT_STRING,
+        }, {
+            .name = "rrsnaptime",
+            .type = QEMU_OPT_NUMBER,
+        }, {
+            .name = "rrsnapcount",
+            .type = QEMU_OPT_NUMBER,
          },
          { /* end of list */ }
      },
diff --git a/qemu-options.hx b/qemu-options.hx
index ac4a30fa83..9eb547bf52 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4651,13 +4651,13 @@ SRST
  ERST
DEF("icount", HAS_ARG, QEMU_OPTION_icount, \ 
-    "-icount 
[shift=N|auto][,align=on|off][,sleep=on|off][,rr=record|replay,rrfile=<filename>[,rrsnapshot=<snapshot>]]\n"
 \
+    "-icount 
[shift=N|auto][,align=on|off][,sleep=on|off][,rr=record|replay,rrfile=<filename>[,rrsnapshot=<snapshot>][,rrsnapmode=initial|periodic][,rrsnaptime=secs][,rrsnapcount=N]\n"
 \
      "                enable virtual instruction counter with 2^N clock ticks 
per\n" \
      "                instruction, enable aligning the host and virtual 
clocks\n" \
      "                or disable real time cpu sleeping, and optionally 
enable\n" \
      "                record-and-replay mode\n", QEMU_ARCH_ALL)
  SRST
-``-icount 
[shift=N|auto][,align=on|off][,sleep=on|off][,rr=record|replay,rrfile=filename[,rrsnapshot=snapshot]]``
+``-icount 
[shift=N|auto][,align=on|off][,sleep=on|off][,rr=record|replay,rrfile=filename[,rrsnapshot=snapshot][,rrsnapmode=initial|periodic][,rrsnaptime=secs][,rrsnapcount=N]]``
      Enable virtual instruction counter. The virtual cpu will execute one
      instruction every 2^N ns of virtual time. If ``auto`` is specified
      then the virtual cpu speed will be automatically adjusted to keep
@@ -4699,6 +4699,11 @@ SRST
      name. In record mode, a new VM snapshot with the given name is created
      at the start of execution recording. In replay mode this option
      specifies the snapshot name used to load the initial VM state.
+    ``rrsnapmode=periodic`` will additionally cause a periodic snapshot to
+    be created after ``rrsnaptime=secs`` seconds of real runtime. The last
+    ``rrsnapcount=N`` periodic snapshots (not including the initial) will
+    be kept (0 for infinite). Periodic snapshots are useful to speed
+    reverse debugging operations near the end of the recorded trace.
  ERST
DEF("watchdog-action", HAS_ARG, QEMU_OPTION_watchdog_action, \
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