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

[Cornelia Huck <cohuck@redhat.com>]

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.

> +
> +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..."

> +
> +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/ ?