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

[Dr. David Alan Gilbert]

* Kirti Wankhede (address@hidden) wrote:
> 
> 
> On 7/11/2019 9:53 PM, Dr. David Alan Gilbert wrote:
> > * 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?
> > 
> 
> I don't have way to test postcopy migration - is one of the major reason
> I had not included postcopy support in this patchset and clearly called
> out in cover letter.
> This patchset is thoroughly tested for precopy migration.
> If anyone have hardware that supports fault, then I would prefer to add
> postcopy support as incremental change later which can be tested before
> submitting.

Agreed; although I think Yan's right to think about how it's going to
work.

> Just a suggestion, instead of using VMSD, is it possible to have some
> additional check to call save_live_complete_precopy from
> qemu_savevm_state_complete_precopy?

Probably yes; although as you can tell the logic in their is already
pretty hairy.

Dave

> 
> >>
> >>>> 2. in each save iteration, how much data is to be saved
> >>>> (https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg04683.html)
> 
> > how big is the data_size ?
> > if this size is too big, it may take too much time and block others.
> 
> I do had mentioned this in the comment about the structure in vfio.h
> header. data_size will be provided by vendor driver and obviously will
> not be greater that migration region size. Vendor driver should be
> responsible to keep its solution optimized.
> 
> 
> >>>> 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)
> 
> I don't think so. Opaque Device data from vendor driver can include
> device state and device memory. Vendor driver who is managing his device
> can decide how to place data over the stream.
> 
> >>>> 4. definition of dirty page copied_pfn
> >>>> (https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg05592.html)
> >>>>
> 
> This was inline to discussion going with Alex. I addressed the concern
> there. Please check current patchset, which addresses the concerns raised.
> 
> >>>> 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:)
> 
> I tried to reply top of threads and addressed common concerns raised in
> that. Sorry If I missed any, I'll make sure to point you to my replies
> going ahead.
> 
> Thanks,
> Kirti
> 
> >>>> 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
> > 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK