Re: [Qemu-devel] [PATCH v4 00/13] Add migration support for VFIO device

[Kirti Wankhede <kwankhede@nvidia.com>]

On 6/21/2019 2:16 PM, Yan Zhao wrote:
> On Fri, Jun 21, 2019 at 04:02:50PM +0800, Kirti Wankhede wrote:
>>
>>
>> On 6/21/2019 6:54 AM, Yan Zhao wrote:
>>> On Fri, Jun 21, 2019 at 08:25:18AM +0800, Yan Zhao wrote:
>>>> On Thu, Jun 20, 2019 at 10:37:28PM +0800, Kirti Wankhede wrote:
>>>>> Add migration support for VFIO device
>>>>>
>>>>> This Patch set include patches as below:
>>>>> - Define KABI for VFIO device for migration support.
>>>>> - Added save and restore functions for PCI configuration space
>>>>> - Generic migration functionality for VFIO device.
>>>>>   * This patch set adds functionality only for PCI devices, but can be
>>>>>     extended to other VFIO devices.
>>>>>   * Added all the basic functions required for pre-copy, stop-and-copy and
>>>>>     resume phases of migration.
>>>>>   * Added state change notifier and from that notifier function, VFIO
>>>>>     device's state changed is conveyed to VFIO device driver.
>>>>>   * During save setup phase and resume/load setup phase, migration region
>>>>>     is queried and is used to read/write VFIO device data.
>>>>>   * .save_live_pending and .save_live_iterate are implemented to use QEMU's
>>>>>     functionality of iteration during pre-copy phase.
>>>>>   * In .save_live_complete_precopy, that is in stop-and-copy phase,
>>>>>     iteration to read data from VFIO device driver is implemented till pending
>>>>>     bytes returned by driver are not zero.
>>>>>   * Added function to get dirty pages bitmap for the pages which are used by
>>>>>     driver.
>>>>> - Add vfio_listerner_log_sync to mark dirty pages.
>>>>> - Make VFIO PCI device migration capable. If migration region is not provided by
>>>>>   driver, migration is blocked.
>>>>>
>>>>> Below is the flow of state change for live migration where states in brackets
>>>>> represent VM state, migration state and VFIO device state as:
>>>>>     (VM state, MIGRATION_STATUS, VFIO_DEVICE_STATE)
>>>>>
>>>>> Live migration save path:
>>>>>         QEMU normal running state
>>>>>         (RUNNING, _NONE, _RUNNING)
>>>>>                         |
>>>>>     migrate_init spawns migration_thread.
>>>>>     (RUNNING, _SETUP, _RUNNING|_SAVING)
>>>>>     Migration thread then calls each device's .save_setup()
>>>>>                         |
>>>>>     (RUNNING, _ACTIVE, _RUNNING|_SAVING)
>>>>>     If device is active, get pending bytes by .save_live_pending()
>>>>>     if pending bytes >= threshold_size,  call save_live_iterate()
>>>>>     Data of VFIO device for pre-copy phase is copied.
>>>>>     Iterate till pending bytes converge and are less than threshold
>>>>>                         |
>>>>>     On migration completion, vCPUs stops and calls .save_live_complete_precopy
>>>>>     for each active device. VFIO device is then transitioned in
>>>>>      _SAVING state.
>>>>>     (FINISH_MIGRATE, _DEVICE, _SAVING)
>>>>>     For VFIO device, iterate in  .save_live_complete_precopy  until
>>>>>     pending data is 0.
>>>>>     (FINISH_MIGRATE, _DEVICE, _STOPPED)
>>>>
>>>> I suggest we also register to VMStateDescription, whose .pre_save
>>>> handler would get called after .save_live_complete_precopy in pre-copy
>>>> only case, and will called before .save_live_iterate in post-copy
>>>> enabled case.
>>>> In the .pre_save handler, we can save all device state which must be
>>>> copied after device stop in source vm and before device start in target vm.
>>>>
>>> hi
>>> to better describe this idea:
>>>
>>> in pre-copy only case, the flow is
>>>
>>> start migration --> .save_live_iterate (several round) -> stop source vm
>>> --> .save_live_complete_precopy --> .pre_save  -->start target vm
>>> -->migration complete
>>>
>>>
>>> in post-copy enabled case, the flow is
>>>
>>> start migration --> .save_live_iterate (several round) --> start post copy --> 
>>> stop source vm --> .pre_save --> start target vm --> .save_live_iterate (several round) 
>>> -->migration complete
>>>
>>> Therefore, we should put saving of device state in .pre_save interface
>>> rather than in .save_live_complete_precopy. 
>>> The device state includes pci config data, page tables, register state, etc.
>>>
>>> The .save_live_iterate and .save_live_complete_precopy should only deal
>>> with saving dirty memory.
>>>
>>
>> Vendor driver can decide when to save device state depending on the VFIO
>> device state set by user. Vendor driver doesn't have to depend on which
>> callback function QEMU or user application calls. In pre-copy case,
>> save_live_complete_precopy sets VFIO device state to
>> VFIO_DEVICE_STATE_SAVING which means vCPUs are stopped and vendor driver
>> should save all device state.
>>
> when post copy stops vCPUs and vfio device, vendor driver only needs to
> provide device state. but how vendor driver knows that, if no extra
> interface or no extra device state is provides?
> 

.save_live_complete_postcopy interface for post-copy will get called,
right?

Thanks,
Kirti

>>>
>>> I know current implementation does not support post-copy. but at least
>>> it should not require huge change when we decide to enable it in future.
>>>
>>
>> .has_postcopy and .save_live_complete_postcopy need to be implemented to
>> support post-copy. I think .save_live_complete_postcopy should be
>> similar to vfio_save_complete_precopy.
>>
>> Thanks,
>> Kirti
>>
>>> Thanks
>>> Yan
>>>