[v5,20/24] replay: simple auto-snapshot mode for record

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(-)

On 18.03.2024 18:46, 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,
> +};
> +typedef enum ReplaySnapshotMode ReplaySnapshotMode;
> +
> +extern ReplaySnapshotMode replay_snapshot_mode;
> +
> +extern uint64_t replay_snapshot_periodic_delay;
> +extern int replay_snapshot_periodic_nr_keep;

Please put the internal variables and enum into the replay-internal.h

> +
>   /* 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);
> +        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;
> +
> +        /* 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 7fd1713fa8..51f2498f12 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4665,13 +4665,13 @@ 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
> @@ -4713,6 +4713,11 @@ 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, \