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

[Yan Zhao <yan.y.zhao@intel.com>]

On Fri, Jun 28, 2019 at 05:44:47PM +0800, Dr. David Alan Gilbert wrote:
> * Yan Zhao (yan.y.zhao@intel.com) wrote:
> > On Tue, Jun 25, 2019 at 03:00:24AM +0800, Dr. David Alan Gilbert wrote:
> > > * Kirti Wankhede (kwankhede@nvidia.com) 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?
> 
> I'm not sure we have a vmsd/pre_save in this setup?  If we did then it's
> a bit confusing because I don't think we have any other iterative device
> that also has a vmsd.
Yes, I tried it. it's ok to register SaveVMHandlers and VMStateDescription at the
same time.

> 
> I'd have to test it, but I think you might get the devices
> ->save_live_complete_precopy called at the right point just before
> postcopy switchover.  It's worth looking.
> 
if a iterative device supports postcopy, then its save_live_complete_precopy
would not get called before postcopy switchover.
However, postcopy may need to save pure device state only data (not memory) at that
time. That's the reason I think we should also register to
VMStateDescription also, as its .pre_save handler would get called at
at that time.

Thanks
Yan

> Dave
> 
> > 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 / dgilbert@redhat.com / Manchester, UK
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK