Re: [PATCH v2 1/1] docs/devel: Add VFIO device migration documentation

["Tarun Gupta (SW-GPU)" <targupta@nvidia.com>]

On 3/15/2021 10:52 PM, Cornelia Huck wrote:
> 
> On Thu, 11 Mar 2021 00:50:09 +0530
> Tarun Gupta <targupta@nvidia.com> wrote:
> 
>> Document interfaces used for VFIO device migration. Added flow of state changes
>> during live migration with VFIO device. Tested by building docs with the new
>> vfio-migration.rst file.
>>
>> v2:
>> - Included the new vfio-migration.rst file in index.rst
>> - Updated dirty page tracking section, also added details about
>>    'pre-copy-dirty-page-tracking' opt-out option.
>> - Incorporated comments around wording of doc.
>>
>> Signed-off-by: Tarun Gupta <targupta@nvidia.com>
>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>> ---
>>   MAINTAINERS                   |   1 +
>>   docs/devel/index.rst          |   1 +
>>   docs/devel/vfio-migration.rst | 135 ++++++++++++++++++++++++++++++++++
>>   3 files changed, 137 insertions(+)
>>   create mode 100644 docs/devel/vfio-migration.rst
>>
> 
> (...)
> 
>> diff --git a/docs/devel/vfio-migration.rst b/docs/devel/vfio-migration.rst
>> new file mode 100644
>> index 0000000000..6196fb132c
>> --- /dev/null
>> +++ b/docs/devel/vfio-migration.rst
>> @@ -0,0 +1,135 @@
>> +=====================
>> +VFIO device Migration
>> +=====================
> 
> Maybe add an introductory sentence or two describing the general
> approach? I.e. we have a general framework, and specific support for
> devices needs to be hooked up.

Ummm, the below paragraph does describe the approach we're using for the 
migration framework involving pre-copy and stop-and-copy phase.
Can you help elaborate more on the general approach you'd like to have?

> 
>> +
>> +VFIO devices use an iterative approach for migration because certain VFIO
>> +devices (e.g. GPU) have large amount of data to be transfered. The iterative
>> +pre-copy phase of migration allows for the guest to continue whilst the VFIO
>> +device state is transferred to the destination, this helps to reduce the total
>> +downtime of the VM. VFIO devices can choose to skip the pre-copy phase of
>> +migration by returning pending_bytes as zero during the pre-copy phase.
> 
> What about something like:
> 
> "Migration of VFIO devices consists of two phases: the optional
> pre-copy phase, and the stop-and-copy phase. The pre-copy phase is
> iterative and allows to accommodate VFIO devices that have a large
> amount of data that needs to be transferred. The iterative pre-copy
> phase..."
> 

Thanks, this looks better. I'll update it in next version incorporating 
the other comments too below.

Thanks,
Tarun

>> +
>> +A detailed description of the UAPI for VFIO device migration can be found in
>> +the comment for the ``vfio_device_migration_info`` structure in the header
>> +file linux-headers/linux/vfio.h.
>> +
>> +VFIO device hooks for iterative approach:
>> +
>> +* A ``save_setup`` function that sets up the migration region, sets _SAVING
>> +  flag in the VFIO device state and informs the VFIO IOMMU module to start
>> +  dirty page tracking.
>> +
>> +* A ``load_setup`` function that sets up the migration region on the
>> +  destination and sets _RESUMING flag in the VFIO device state.
>> +
>> +* A ``save_live_pending`` function that reads pending_bytes from the vendor
>> +  driver, which indicates the amount of data that the vendor driver has yet to
>> +  save for the VFIO device.
>> +
>> +* A ``save_live_iterate`` function that reads the VFIO device's data from the
>> +  vendor driver through the migration region during iterative phase.
>> +
>> +* A ``save_live_complete_precopy`` function that resets _RUNNING flag from the
>> +  VFIO device state, saves the device config space, if any, and iteratively
>> +  copies the remaining data for the VFIO device untill the vendor driver
> 
> s/untill/until/
> 
>> +  indicates that no data remains (pending bytes is zero).
>> +
>> +* A ``load_state`` function that loads the config section and the data
>> +  sections that are generated by the save functions above
>> +
>> +* ``cleanup`` functions for both save and load that perform any migration
>> +  related cleanup, including unmapping the migration region
>> +
>> +A VM state change handler is registered to change the VFIO device state when
>> +the VM state changes.
>> +
>> +Similarly, a migration state change notifier is registered to get a
>> +notification on migration state change. These states are translated to VFIO
> 
> s/to/to the corresponding/
> 
>> +device state and conveyed to vendor driver.
> 
> s/to/to the/
> 
> (...)
> 
>> +Postcopy
>> +========
>> +
>> +Postcopy migration is not supported for VFIO devices.
> 
> s/is not/is currently not/ ?
>