Re: [Qemu-devel] [PATCH 5/7] migration: create now 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.

A couple of questions:
   1) Does the ordering of loading this state matter - lets say that the source
     was in an error state, then all the other device states get loaded and then
     it loads your global_state which tells the destination is in error - is that
     too late ? Could the device emulation have already started doing some IO
     or something to the devices which it wouldn't have done if it knew there was
     already a problem?

   2) What's the advantage of the optional section over using the 'command' sections
      I use in postcopy; http://lists.gnu.org/archive/html/qemu-devel/2014-10/msg00337.html ?

Dave

> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---
>  include/migration/migration.h |  4 ++
>  migration.c                   | 88 +++++++++++++++++++++++++++++++++++++++++--
>  vl.c                          |  1 +
>  3 files changed, 90 insertions(+), 3 deletions(-)
> 
> diff --git a/include/migration/migration.h b/include/migration/migration.h
> index 3cb5ba8..bc1069b 100644
> --- a/include/migration/migration.h
> +++ b/include/migration/migration.h
> @@ -174,4 +174,8 @@ size_t ram_control_save_page(QEMUFile *f, ram_addr_t block_offset,
>                               ram_addr_t offset, size_t size,
>                               int *bytes_sent);
> 
> +void register_global_state(void);
> +void global_state_store(void);
> +char *global_state_get_runstate(void);
> +
>  #endif
> diff --git a/migration.c b/migration.c
> index 8d675b3..6f7e50e 100644
> --- a/migration.c
> +++ b/migration.c
> @@ -112,10 +112,20 @@ 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);
> +        }
>      }
>  }
> 
> @@ -608,6 +618,7 @@ 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);
> @@ -699,3 +710,74 @@ void migrate_fd_connect(MigrationState *s)
>      qemu_thread_create(&s->thread, "migration", migration_thread, s,
>                         QEMU_THREAD_JOINABLE);
>  }
> +
> +typedef struct {
> +    int32_t size;
> +    uint8_t runstate[100];
> +} GlobalState;
> +
> +static GlobalState global_state;
> +
> +void global_state_store(void)
> +{
> +    if (runstate_store((char*)global_state.runstate,
> +                       sizeof(global_state.runstate)) == -1) {
> +        printf("Runstate is too big\n");
> +        exit(-1);
> +    }
> +}
> +
> +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);
> +
> +    if (strcmp(runstate, "running") != 0) {
> +
> +        RunState r = runstate_index(runstate);
> +
> +        if (r == -1) {
> +            printf("Unknown received state %s\n", runstate);
> +            return -1;
> +        }
> +        ret = vm_stop_force_state(r);
> +    }
> +
> +   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);
> +}
> +
> +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);
> +}
> diff --git a/vl.c b/vl.c
> index 1788b6a..75e855e 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -4511,6 +4511,7 @@ 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);
> -- 
> 2.1.0
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK