Re: [Qemu-devel] [PATCH v2 3/6] migration: move global_state.optional out

[Juan Quintela]

Eduardo Habkost <address@hidden> wrote:
D> On Fri, Jun 09, 2017 at 11:48:59AM +0800, Peter Xu wrote:
>> Put it into MigrationState then we can use the properties to specify
>> whether to enable storing global state.
>> 
>> Removing global_state_set_optional() since now we can use HW_COMPAT_2_3
>> for x86/power in general, and the register_compat_prop() for xen_init().
>> 
>> Signed-off-by: Peter Xu <address@hidden>
>> ---
>>  hw/i386/pc_piix.c             |  1 -
>>  hw/ppc/spapr.c                |  1 -
>>  hw/xen/xen-common.c           |  8 +++++++-
>>  include/hw/compat.h           |  4 ++++
>>  include/migration/migration.h |  7 ++++++-
>>  migration/migration.c         | 24 ++++++++++++++++--------
>>  6 files changed, 33 insertions(+), 12 deletions(-)
>> 
>> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
>> index 2234bd0..c83cec5 100644
>> --- a/hw/i386/pc_piix.c
>> +++ b/hw/i386/pc_piix.c
>> @@ -317,7 +317,6 @@ static void pc_compat_2_3(MachineState *machine)
>>      if (kvm_enabled()) {
>>          pcms->smm = ON_OFF_AUTO_OFF;
>>      }
>> -    global_state_set_optional();
>>      savevm_skip_configuration();
>>  }
>>  
>> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
>> index ab3aab1..3e78bb9 100644
>> --- a/hw/ppc/spapr.c
>> +++ b/hw/ppc/spapr.c
>> @@ -3593,7 +3593,6 @@ static void 
>> spapr_machine_2_3_instance_options(MachineState *machine)
>>  {
>>      spapr_machine_2_4_instance_options(machine);
>>      savevm_skip_section_footers();
>> -    global_state_set_optional();
>>      savevm_skip_configuration();
>>  }
>
> This is a good thing: makes the migration behavior of the
> machine-types introspectable in compat_props.
>
> I suggest moving this part (and all the rest except the new
> register_compat_prop() call below) to a separate patch, because
> it is an improvement on its own.
>
>>  
>> diff --git a/hw/xen/xen-common.c b/hw/xen/xen-common.c
>> index 0bed577..8240d50 100644
>> --- a/hw/xen/xen-common.c
>> +++ b/hw/xen/xen-common.c
>> @@ -138,7 +138,13 @@ static int xen_init(MachineState *ms)
>>      }
>>      qemu_add_vm_change_state_handler(xen_change_state_handler, NULL);
>>  
>> -    global_state_set_optional();
>> +    /*
>> +     * TODO: make sure global MigrationState has not yet been created
>> +     * (otherwise the compat trick won't work). For now we are in
>> +     * configure_accelerator() so we are mostly good. Better to
>> +     * confirm that in the future.
>> +     */
>
> So, this makes accelerator initialization very sensitive to
> ordering.
>
>> +    register_compat_prop("migration", "store-global-state", "off");
>
> It's already hard to track down machine-type stuff that is
> initialized at machine->init() time but it's not introspectable,
> let's not do the same mistake with accelerators.
>
> Can't this be solved by a AcceleratorClass::global_props field,
> so we are sure there's a single place in the code where globals
> are registered?  (Currently, they are all registered by the few
> lines around the machine_register_compat_props() call in main()).
>
> I think I see other use cases for a new
> AcceleratorClass::global_props field.  e.g.: replacing
> kvm_default_props and tcg_default_props in target/i386/cpu.c.

That is the kind of discussion that I wanted to start.  Thanks.
For the migration point of view, we don't care *how* or *where* it is
implemented, just that it is easy to add new properties (call it
whatever name that you want).

And yes, making xen/kvm/tcg use QOM like everything else looks like a
good idea to me.

Later, Juan.