Re: [Qemu-devel] [PATCH v7 00/13] Add migration support for VFIO device

[Dr. David Alan Gilbert]

* Yan Zhao (address@hidden) wrote:
> On Thu, Jul 11, 2019 at 06:50:12PM +0800, Dr. David Alan Gilbert wrote:
> > * Yan Zhao (address@hidden) wrote:
> > > Hi Kirti,
> > > There are still unaddressed comments to your patches v4.
> > > Would you mind addressing them?
> > > 
> > > 1. should we register two migration interfaces simultaneously
> > > (https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg04750.html)
> > 
> > Please don't do this.
> > As far as I'm aware we currently only have one device that does that
> > (vmxnet3) and a patch has just been posted that fixes/removes that.
> > 
> > Dave
> >
> hi Dave,
> Thanks for notifying this. but if we want to support postcopy in future,
> after device stops, what interface could we use to transfer data of
> device state only?
> for postcopy, when source device stops, we need to transfer only
> necessary device state to target vm before target vm starts, and we
> don't want to transfer device memory as we'll do that after target vm
> resuming.

Hmm ok, lets see; that's got to happen in the call to:
    qemu_savevm_state_complete_precopy(fb, false, false);
that's made from postcopy_start.
 (the false's are iterable_only and inactivate_disks)

and at that time I believe the state is POSTCOPY_ACTIVE, so in_postcopy
is true.

If you're doing postcopy, then you'll probably define a has_postcopy()
function, so qemu_savevm_state_complete_precopy will skip the
save_live_complete_precopy call from it's loop for at least two of the
reasons in it's big if.

So you're right; you need the VMSD for this to happen in the second
loop in qemu_savevm_state_complete_precopy.  Hmm.

Now, what worries me, and I don't know the answer, is how the section
header for the vmstate and the section header for an iteration look
on the stream; how are they different?

Dave

> Thanks
> Yan
> 
> > > 2. in each save iteration, how much data is to be saved
> > > (https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg04683.html)
> > > 3. do we need extra interface to get data for device state only
> > > (https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg04812.html)
> > > 4. definition of dirty page copied_pfn
> > > (https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg05592.html)
> > > 
> > > Also, I'm glad to see that you updated code by following my comments 
> > > below,
> > > but please don't forget to reply my comments next time:)
> > > https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg05357.html
> > > https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg06454.html
> > > 
> > > Thanks
> > > Yan
> > > 
> > > On Tue, Jul 09, 2019 at 05:49:07PM +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)
> > > >                         |
> > > >     (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.
> > > > 
> > > > v6 -> v7:
> > > > - Fix build failures.
> > > > 
> > > > v5 -> v6:
> > > > - Fix build failure.
> > > > 
> > > > v4 -> v5:
> > > > - Added decriptive comment about the sequence of access of members of 
> > > > structure
> > > >   vfio_device_migration_info to be followed based on Alex's suggestion
> > > > - Updated get dirty pages sequence.
> > > > - As per Cornelia Huck's suggestion, added callbacks to VFIODeviceOps to
> > > >   get_object, save_config and load_config.
> > > > - Fixed multiple nit picks.
> > > > - Tested live migration with multiple vfio device assigned to a VM.
> > > > 
> > > > 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 vfio_get_object callback to VFIODeviceOps
> > > >   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.
> > > > 
> > > >  hw/vfio/Makefile.objs         |   2 +-
> > > >  hw/vfio/common.c              |  55 +++
> > > >  hw/vfio/migration.c           | 874 
> > > > ++++++++++++++++++++++++++++++++++++++++++
> > > >  hw/vfio/pci.c                 | 137 ++++++-
> > > >  hw/vfio/trace-events          |  19 +
> > > >  include/hw/vfio/vfio-common.h |  25 ++
> > > >  linux-headers/linux/vfio.h    | 166 ++++++++
> > > >  7 files changed, 1271 insertions(+), 7 deletions(-)
> > > >  create mode 100644 hw/vfio/migration.c
> > > > 
> > > > -- 
> > > > 2.7.0
> > > > 
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK