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

[Yan Zhao]

On Tue, Jun 25, 2019 at 03:00:24AM +0800, Dr. David Alan Gilbert wrote:
> * Kirti Wankhede (address@hidden) wrote:
> > 
> > 
> > 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?
> 
> That happens at the very end; I think the question here is for something
> that gets called at the point we stop iteratively sending RAM, send the
> device states and then start sending RAM on demand to the destination
> as it's running. Typically we send a small set of device state
> (registers etc) at this point.
> 
> I guess there's two different postcopy cases that we need to think
> about:
>   a) Where the VFIO device doesn't support postcopy - it just gets
>   migrated like any other device, so all it's RAM must get sent
>   before we flip into postcopy mode.
> 
>   b) Where the VFIO device does support postcopy - where the pages
>   get sent on demand.
> 
> (b) maybe tricky depending on whether your hardware can fault
> on pages of your RAM that are needed but not yet transferred;  but
> if you can that would make life a lot more practical on really
> big VFO devices.
> 
> Dave
>
hi Dave,
so do you think it is good to abstract device state data and save it in
.pre_save callback?

Thanks
Yan

> > 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
> > >>>
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK