Re: [Qemu-devel] [PATCH 03/11] migration: create new section to store global state

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Juan Quintela (quintela@redhat.com) wrote:
> This includes a new section that for now just stores the current qemu state.
> 
> Right now, there are only one way to control what is the state of the
> target after migration.
> 
> - If you run the target qemu with -S, it would start stopped.
> - If you run the target qemu without -S, it would run just after migration finishes.
> 
> The problem here is what happens if we start the target without -S and
> there happens one error during migration that puts current state as
> -EIO.  Migration would ends (notice that the error happend doing block
> IO, network IO, i.e. nothing related with migration), and when
> migration finish, we would just "continue" running on destination,
> probably hanging the guest/corruption data, whatever.
> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---
>  include/migration/migration.h |  1 +
>  migration/migration.c         | 93 +++++++++++++++++++++++++++++++++++++++++--
>  vl.c                          |  1 +
>  3 files changed, 92 insertions(+), 3 deletions(-)
> 
> diff --git a/include/migration/migration.h b/include/migration/migration.h
> index 9387c8c..1280193 100644
> --- a/include/migration/migration.h
> +++ b/include/migration/migration.h
> @@ -197,4 +197,5 @@ size_t ram_control_save_page(QEMUFile *f, ram_addr_t block_offset,
> 
>  void ram_mig_init(void);
>  void savevm_skip_section_footers(void);
> +void register_global_state(void);
>  #endif
> diff --git a/migration/migration.c b/migration/migration.c
> index b04b457..01bb90d 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -25,6 +25,7 @@
>  #include "qemu/thread.h"
>  #include "qmp-commands.h"
>  #include "trace.h"
> +#include "qapi/util.h"
> 
>  #define MAX_THROTTLE  (32 << 20)      /* Migration speed throttling */
> 
> @@ -96,6 +97,81 @@ void migration_incoming_state_destroy(void)
>      mis_current = NULL;
>  }
> 
> +
> +typedef struct {
> +    int32_t size;

Still think that size should be unsigned.

> +    uint8_t runstate[100];
> +} GlobalState;
> +
> +static GlobalState global_state;
> +
> +static void global_state_store(void)
> +{
> +    if (!runstate_store((char *)global_state.runstate,
> +                        sizeof(global_state.runstate))) {
> +        printf("Runstate is too big\n");
> +        exit(-1);

Hmmmm:
  1) Shouldn't that be an error report? or an assert?
  2) Exit should use EXIT_FAILURE (which is +ve as well)
  3) But you shouldn't kill the guest on an outwards migration
     - just fail the migration.
  4) (And anyway this all seems overkill for sending a 
      status string).

> +    }
> +}
> +
> +static char *global_state_get_runstate(void)
> +{
> +    return (char *)global_state.runstate;
> +}
> +
> +static int global_state_post_load(void *opaque, int version_id)
> +{
> +    GlobalState *s = opaque;
> +    int ret = 0;
> +    char *runstate = (char *)s->runstate;
> +
> +    printf("loaded state: %s\n", runstate);

trace_.....

> +
> +    if (strcmp(runstate, "running") != 0) {
> +        Error *local_err = NULL;
> +        int r = qapi_enum_parse(RunState_lookup, runstate, RUN_STATE_MAX,
> +                                -1, &local_err);

Is there a reason not to do the qapi_enum_parse first, and then
compare it's output to RUN_STATE_RUNNING?

> +
> +        if (r == -1) {
> +            if (local_err) {
> +                error_report_err(local_err);
> +            }
> +            return -1;

I'm not sure, but shouldn't that be -EINVAL ?
(Not that vmstate is consistent about it)

> +        }
> +        ret = vm_stop_force_state(r);

Kind of going back to adding the state transitions;
don't you need to allow INMIGRATE to * - because it could
be pretty much anything? (Shutdown? Suspended?)

> +    }
> +
> +   return ret;
> +}
> +
> +static void global_state_pre_save(void *opaque)
> +{
> +    GlobalState *s = opaque;
> +
> +    s->size = strlen((char *)s->runstate) + 1;
> +    printf("saved state: %s\n", s->runstate);

trace_

> +}
> +
> +static const VMStateDescription vmstate_globalstate = {
> +    .name = "globalstate",
> +    .version_id = 1,
> +    .minimum_version_id = 1,
> +    .post_load = global_state_post_load,
> +    .pre_save = global_state_pre_save,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_INT32(size, GlobalState),
> +        VMSTATE_BUFFER(runstate, GlobalState),
> +        VMSTATE_END_OF_LIST()
> +    },
> +};
> +
> +void register_global_state(void)
> +{
> +    /* We would use it independently that we receive it */
> +    strcpy((char *)&global_state.runstate, "");
> +    vmstate_register(NULL, 0, &vmstate_globalstate, &global_state);
> +}
> +
>  /*
>   * Called on -incoming with a defer: uri.
>   * The migration can be started later after any parameters have been
> @@ -163,10 +239,20 @@ static void process_incoming_migration_co(void *opaque)
>          exit(EXIT_FAILURE);
>      }
> 
> -    if (autostart) {
> +    /* runstate == "" means that we haven't received it through the
> +     * wire, so we obey autostart.  runstate == runing means that we
> +     * need to run it, we need to make sure that we do it after
> +     * everything else has finished.  Every other state change is done
> +     * at the post_load function */
> +
> +    if (strcmp(global_state_get_runstate(), "running") == 0) {
>          vm_start();
> -    } else {
> -        runstate_set(RUN_STATE_PAUSED);
> +    } else if (strcmp(global_state_get_runstate(), "") == 0) {
> +        if (autostart) {
> +            vm_start();
> +        } else {
> +            runstate_set(RUN_STATE_PAUSED);
> +        }
>      }
>      migrate_decompress_threads_join();
>  }
> @@ -791,6 +877,7 @@ static void *migration_thread(void *opaque)
>                  qemu_system_wakeup_request(QEMU_WAKEUP_REASON_OTHER);
>                  old_vm_running = runstate_is_running();
> 
> +                global_state_store();
>                  ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
>                  if (ret >= 0) {
>                      qemu_file_set_rate_limit(s->file, INT64_MAX);
> diff --git a/vl.c b/vl.c
> index 555fd88..95acdb1 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -4473,6 +4473,7 @@ int main(int argc, char **argv, char **envp)
>          return 0;
>      }
> 
> +    register_global_state();
>      if (incoming) {
>          Error *local_err = NULL;
>          qemu_start_incoming_migration(incoming, &local_err);
> -- 
> 2.4.3
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK