Re: [PATCH] vfio/migrate: Move switch of dirty tracking into vfio_memory_listener

[Paolo Bonzini <pbonzini@redhat.com>]

On 28/01/21 21:02, Dr. David Alan Gilbert wrote:
> * Paolo Bonzini (pbonzini@redhat.com) wrote:
>> On 11/01/21 08:34, Keqian Zhu wrote:
>>> +static void vfio_listener_log_start(MemoryListener *listener,
>>> +                                    MemoryRegionSection *section,
>>> +                                    int old, int new)
>>> +{
>>> +    VFIOContainer *container = container_of(listener, VFIOContainer, listener);
>>> +
>>> +    vfio_set_dirty_page_tracking(container, true);
>>> +}
>>
>> This would enable dirty page tracking also just for having a framebuffer
>> (DIRTY_MEMORY_VGA).  Technically it would be correct, but it would also be
>> more heavyweight than expected.
> 
> Wouldn't that only happen on emulated video devices?

Yes, but still it's not impossible to have both an emulated VGA and an 
assigned GPU or vGPU.

>> In order to only cover live migration, you can use the log_global_start and
>> log_global_stop callbacks instead.
>>
>> If you want to use log_start and log_stop, you need to add respectively
>>
>>      if (old != 0) {
>>          return;
>>      }
>>
>> and
>>
>>      if (new != 0) {
>>          return;
>>      }
> 
> Why 0, wouldn't you be checking for DIRTY_LOG_MIGRATION somewhere?

Actually thinking more about it log_start/log_stop are just wrong, 
because they would be called many times, for each MemoryRegionSection.

Paolo