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

[Yan Zhao]

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.


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.

Thanks
Yan

> >                         |
> >     (FINISH_MIGRATE, _COMPLETED, STOPPED)
> >     Migraton thread schedule cleanup bottom half and exit
> > 
> > Live migration resume path:
> >     Incomming migration calls .load_setup for each device
> >     (RESTORE_VM, _ACTIVE, STOPPED)
> >                         |
> >     For each device, .load_state is called for that device section data
> >                         |
> >     At the end, called .load_cleanup for each device and vCPUs are started.
> >                         |
> >         (RUNNING, _NONE, _RUNNING)
> > 
> > Note that:
> > - Migration post copy is not supported.
> > 
> > v3 -> v4:
> > - Added one more bit for _RESUMING flag to be set explicitly.
> > - data_offset field is read-only for user space application.
> > - data_size is read for every iteration before reading data from migration, 
> > that
> >   is removed assumption that data will be till end of migration region.
> > - If vendor driver supports mappable sparsed region, map those region during
> >   setup state of save/load, similarly unmap those from cleanup routines.
> > - Handles race condition that causes data corruption in migration region 
> > during
> >   save device state by adding mutex and serialiaing save_buffer and
> >   get_dirty_pages routines.
> > - Skip called get_dirty_pages routine for mapped MMIO region of device.
> > - Added trace events.
> > - Splitted into multiple functional patches.
> > 
> > v2 -> v3:
> > - Removed enum of VFIO device states. Defined VFIO device state with 2 bits.
> > - Re-structured vfio_device_migration_info to keep it minimal and defined 
> > action
> >   on read and write access on its members.
> > 
> > v1 -> v2:
> > - Defined MIGRATION region type and sub-type which should be used with 
> > region
> >   type capability.
> > - Re-structured vfio_device_migration_info. This structure will be placed 
> > at 0th
> >   offset of migration region.
> > - Replaced ioctl with read/write for trapped part of migration region.
> > - Added both type of access support, trapped or mmapped, for data section 
> > of the
> >   region.
> > - Moved PCI device functions to pci file.
> > - Added iteration to get dirty page bitmap until bitmap for all requested 
> > pages
> >   are copied.
> > 
> > Thanks,
> > Kirti
> > 
> > 
> > Kirti Wankhede (13):
> >   vfio: KABI for migration interface
> >   vfio: Add function to unmap VFIO region
> >   vfio: Add save and load functions for VFIO PCI devices
> >   vfio: Add migration region initialization and finalize function
> >   vfio: Add VM state change handler to know state of VM
> >   vfio: Add migration state change notifier
> >   vfio: Register SaveVMHandlers for VFIO device
> >   vfio: Add save state functions to SaveVMHandlers
> >   vfio: Add load state functions to SaveVMHandlers
> >   vfio: Add function to get dirty page list
> >   vfio: Add vfio_listerner_log_sync to mark dirty pages
> >   vfio: Make vfio-pci device migration capable.
> >   vfio: Add trace events in migration code path
> > 
> >  hw/vfio/Makefile.objs         |   2 +-
> >  hw/vfio/common.c              |  55 +++
> >  hw/vfio/migration.c           | 815 
> > ++++++++++++++++++++++++++++++++++++++++++
> >  hw/vfio/pci.c                 | 126 ++++++-
> >  hw/vfio/pci.h                 |  29 ++
> >  hw/vfio/trace-events          |  19 +
> >  include/hw/vfio/vfio-common.h |  22 ++
> >  linux-headers/linux/vfio.h    |  71 ++++
> >  8 files changed, 1132 insertions(+), 7 deletions(-)
> >  create mode 100644 hw/vfio/migration.c
> > 
> > -- 
> > 2.7.0
> >