Re: [Qemu-devel] [PATCH 2/4] vl: move global property, migrate init earlier

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Jul 03, 2017 at 10:44:06AM +0800, Peter Xu wrote:
> Currently drive_init_func() may call migrate_get_current() while the
> migrate object is still not ready yet at that time. Move the migration
> object init earlier, along with the global properties, right after
> acceleration init.
> 
> This fixes a breakage for iotest 055, which caused an assertion failure.
> 
> Reported-by: Max Reitz <mreitz@redhat.com>
> Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> Fixes: 3df663 ("migration: move only_migratable to MigrationState")
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  vl.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/vl.c b/vl.c
> index 0c497a3..2ae4313 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -4414,6 +4414,18 @@ int main(int argc, char **argv, char **envp)
>  
>      configure_accelerator(current_machine);
>  
> +    /*
> +     * Register all the global properties, including accel properties,
> +     * machine properties, and user-specified ones.
> +     */
> +    register_global_properties(current_machine);
> +
> +    /*
> +     * Migration object can only be created after global properties
> +     * are applied correctly.
> +     */
> +    migration_object_init();
> +

So, things that might introduce bugs here are:
1) Unexpected qdev_prop_register_global() calls between this place and
   the original register_global_properties() call (that would now happen
   in a different order).
2) register_global_properties() seeing a different global property list
   because it is being called earlier.
   2.1) AccelClass::global_props is statically defined and will be the
        same here. Not a problem.
   2.2) MachineClass::compat_props: same as above.
   2.3) user-provided global properties: we need to ensure all
        properties in the "global" QemuOpts section are already
        available at this point.


To ensure (1) is not a problem, we need to check all calls for
qdev_prop_register_global().  The callers are:

* configure_rtc()
  - Called very early, when parsing command-line options.  Not a problem.[1]
* global_init_func()
  * Called by user_register_global_props()
    * Called by register_global_properties().
      - That's the code we're moving.  Not a problem.
* QEMU_OPTION_rtc_td_hack case in main()
  - Called very early, when parsing command-line options.  Not a problem.[1]
* QEMU_OPTION_no_kvm_pit_reinjection case in main()
  - Called very early, when parsing command-line options.  Not a problem.[1]
* register_compat_prop()
  * Called by machine_register_compat_props()
    * Called by register_global_properties().
      - That's the code we're moving.  Not a problem.
  * Called by machine_register_compat_for_subclass()
    * Called by machine_register_compat_props() (see above)
  * Called by register_compat_props_array()
    * Called by accel_register_compat_props()
      * Called by register_global_properties().
        - That's the code we're moving.  Not a problem.
* qdev_prop_register_global_list()
  - Used only by unit test code.
* cpu_common_parse_features()
  - Used when initializing CPUs, which is done much later, when
    machine_run_board_init() is called (or when -device is handled by
    device_init_func()).  Not a problem.[2]
* x86_cpu_parse_featurestr()
  - Same as above.


To ensure (2.3) is not a problem, we need to look for references to
qemu_find_opts("global") or qemu_global_opts.  They are:

* user_register_global_props()
  - This is the code we're moving.  Not a problem.
* default_driver_check() call at main()
  - This happens earlier, before the code we're moving.  Not a problem.
* qemu_global_option()
  - Called very early, when parsing command-line options.  Not a problem.


So the code reordering looks OK.

Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>


Notes about things we need to fix in the future:

[1] I think they should be replaced by qemu_global_option() calls, though.
[2] This is a fragile portion of the global property code and should be
    eventually moved to the command-line parsing section of main().


>      if (qtest_chrdev) {
>          qtest_init(qtest_chrdev, qtest_log, &error_fatal);
>      }
> @@ -4595,18 +4607,6 @@ int main(int argc, char **argv, char **envp)
>              exit (i == 1 ? 1 : 0);
>      }
>  
> -    /*
> -     * Register all the global properties, including accel properties,
> -     * machine properties, and user-specified ones.
> -     */
> -    register_global_properties(current_machine);
> -
> -    /*
> -     * Migration object can only be created after global properties
> -     * are applied correctly.
> -     */
> -    migration_object_init();
> -
>      /* This checkpoint is required by replay to separate prior clock
>         reading from the other reads, because timer polling functions query
>         clock values from the log. */
> -- 
> 2.7.4
> 

-- 
Eduardo