Re: [PATCH v3 1/1] docs/devel: Add VFIO device migration documentation

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

On 4/1/2021 4:35 PM, Cornelia Huck wrote:
> 
> On Fri, 26 Mar 2021 18:48:50 +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.
> 
> I don't think you want to include the test state in the patch
> description; that should go into a --- section that is stripped off by
> git am.
> 
>>
>> v3:
>> - Add introductory line about VM migration in general.
>> - Remove occurcences of vfio_pin_pages() to describe pinning.
>> - Incorporated comments from v2
>>
>> 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.
> 
> Same for the changelog; this is interesting for review, but not for the
> final git log.
>

Will move these details in --- section.

>>
>> Signed-off-by: Tarun Gupta <targupta@nvidia.com>
>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> 
> This S-o-b chain does not look correct. Your address should be the last
> one in the chain, signing off on all of the previous ones. (Maybe Kirti
> also needs to be listed in a Co-developed-by: statement?)
> 
>> ---
>>   MAINTAINERS                   |   1 +
>>   docs/devel/index.rst          |   1 +
>>   docs/devel/vfio-migration.rst | 143 ++++++++++++++++++++++++++++++++++
>>   3 files changed, 145 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..24cb55991a
>> --- /dev/null
>> +++ b/docs/devel/vfio-migration.rst
> 
> (...)
> 
>> +VFIO device hooks for iterative approach:
> 
> "VFIO implements the device hooks for the iterative approach as
> follows:"
> 
> ?
> 
>> +
>> +* 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 until the vendor driver
>> +  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.
> 
> This sentence is not very informative. What about:
> 
> "The VFIO migration code uses a VM state change handler to change the
> VFIO device state when the VM state changes from running to
> not-running, and vice versa."
> 
>> +
>> +Similarly, a migration state change notifier is registered to get a
>> +notification on migration state change. These states are translated to the
>> +corresponding VFIO device state and conveyed to the vendor driver.
> 
> "Similarly, a migration state change handler is used to transition the
> VFIO device state back to _RUNNING in case a migration failed or was
> canceled."

I wanted to keep the statement generic because the VFIO device state can 
be _RUNNING, _SAVING, _RESUMING. I can use your statement as an example 
as to how the migration state can be changed back to _RUNNING in case of 
migration failure or cancel. Does that work?

Thanks,
Tarun

> 
> 
>> +
>> +System memory dirty pages tracking
>> +----------------------------------
>> +
>> +A ``log_sync`` memory listener callback marks those system memory pages
>> +as dirty which are used for DMA by the VFIO device. The dirty pages bitmap is
>> +queried per container. All pages pinned by the vendor driver through external
>> +APIs have to be marked as dirty during migration. When there are CPU writes,
>> +CPU dirty page tracking can identify dirtied pages, but any page pinned by the
>> +vendor driver can also be written by device. There is currently no device or
> 
> s/by/by the/
> 
>> +IOMMU support for dirty page tracking in hardware.
>> +
>> +By default, dirty pages are tracked when the device is in pre-copy as well as
>> +stop-and-copy phase. So, a page pinned by vendor driver will be copied to
> 
> s/by/by the/
> s/to/to the/
> 
>> +destination in both the phases. Copying dirty pages in pre-copy phase helps
> 
> s/both the/both/ ?
> 
>> +QEMU to predict if it can achieve its downtime tolerances. If QEMU during
>> +pre-copy phase keeps finding dirty pages continuously, then it understands
>> +that even in stop-and-copy phase, it is likely to find dirty pages and can
>> +predict the downtime accordingly
>> +
>> +QEMU also provides per device opt-out option ``pre-copy-dirty-page-tracking``
> 
> s/provides/provides a/
> 
>> +which disables querying dirty bitmap during pre-copy phase. If it is set to
> 
> s/querying/querying the/
> 
>> +off, all dirty pages will be copied to destination in stop-and-copy phase only
> 
> s/to/to the/
> 
> (...)
>