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

From: Kirti Wankhede
Subject: Re: [Qemu-devel] [PATCH v7 00/13] Add migration support for VFIO device
Date: Fri, 12 Jul 2019 00:38:31 +0530

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

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

>>>> 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.


>>>> 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:
>>>>> Live migration save path:
>>>>>         QEMU normal running state
>>>>>         (RUNNING, _NONE, _RUNNING)
>>>>>                         |
>>>>>     migrate_init spawns migration_thread.
>>>>>     Migration thread then calls each device's .save_setup()
>>>>>                         |
>>>>>     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.
>>>>>     For VFIO device, iterate in  .save_live_complete_precopy  until
>>>>>     pending data is 0.
>>>>>                         |
>>>>>     Migraton thread schedule cleanup bottom half and exit
>>>>> Live migration resume path:
>>>>>     Incomming migration calls .load_setup for each device
>>>>>                         |
>>>>>     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

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