Re: [Qemu-devel] [PATCH 06/12] kvmvapic: fixing loading vmstate

[Paolo Bonzini <pbonzini@redhat.com>]

Il 09/09/2014 12:30, Pavel Dovgaluk ha scritto:
>> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
>> Il 27/08/2014 15:03, Pavel Dovgaluk ha scritto:
>>>>> Hmm, probably not.  The bug would not be other timers accessing the
>>>>> APIC, because that would also call apic_sync_vapic and the only effect
>>>>> would be an extra useless synchronization.  The bug would happen if the
>>>>> APIC is accessed by the CPU before the timer has the occasion to run.
>>> Sorry, but I don't understand which problem we will solve with apic_sync_vapic.
>>
>> Taking inspiration from what KVM does, the fix could be even simpler
>> than a change state handler.  run_on_cpu functions do not run while the
>> VM is stopped, so the following should work:
>>
>> diff --git a/hw/intc/apic_common.c b/hw/intc/apic_common.c
>> index ce3d903..81d1ad7 100644
>> --- a/hw/intc/apic_common.c
>> +++ b/hw/intc/apic_common.c
>> @@ -91,13 +91,20 @@ void apic_enable_tpr_access_reporting(DeviceState
>> *dev, bool enable)
>>      }
>>  }
>>
>> +static void do_apic_enable_vapic(void *data)
>> +{
>> +    APICCommonState *s = APIC_COMMON(data);
>> +    APICCommonClass *info = APIC_COMMON_GET_CLASS(s);
>> +
>> +    info->vapic_base_update(s);
>> +}
>> +
>>  void apic_enable_vapic(DeviceState *dev, hwaddr paddr)
>>  {
>>      APICCommonState *s = APIC_COMMON(dev);
>> -    APICCommonClass *info = APIC_COMMON_GET_CLASS(s);
>>
>>      s->vapic_paddr = paddr;
>> -    info->vapic_base_update(s);
>> +    run_on_cpu(CPU(s->cpu), do_apic_enable_vapic, s);
>>  }
> 
> I've tried this one and it doesn't work.
> do_apic_enable_vapic runs on cpu, at the same time the VM state is loaded.
> APIC state still remains broken because of this.

You're right (in fact run_on_cpu is synchronous so the alternative would
have been deadlock).  A change state handler can work.  I'll submit your
patches to the migration maintainers, including an alternative fix for
this VAPIC problem.

Paolo