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

[Tian, Kevin]

> From: Tarun Gupta <targupta@nvidia.com>
> Sent: Thursday, March 11, 2021 3:20 AM
> 
> 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/MAINTAINERS b/MAINTAINERS
> index 738786146d..a2a80eee59 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -1801,6 +1801,7 @@ M: Alex Williamson <alex.williamson@redhat.com>
>  S: Supported
>  F: hw/vfio/*
>  F: include/hw/vfio/
> +F: docs/devel/vfio-migration.rst
> 
>  vfio-ccw
>  M: Cornelia Huck <cohuck@redhat.com>
> diff --git a/docs/devel/index.rst b/docs/devel/index.rst
> index ae664da00c..5330f1ca1d 100644
> --- a/docs/devel/index.rst
> +++ b/docs/devel/index.rst
> @@ -39,3 +39,4 @@ Contents:
>     qom
>     block-coroutine-wrapper
>     multi-process
> +   vfio-migration
> 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
> +=====================
> +
> +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.
> +
> +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

and if any, 

> +  copies the remaining data for the VFIO device untill 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.
> +
> +Similarly, a migration state change notifier is registered to get a
> +notification on migration state change. These states are translated to VFIO
> +device state and conveyed to vendor driver.
> +
> +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
> +vfio_pin_pages() external API have to be marked as dirty during migration.

why mention kernel internal functions in an userspace doc?

> +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 which has hardware support for dirty page tracking.

no device or IOMMU support

> So
> +all pages which are pinned by vendor driver are considered as dirty.

Similarly, why do we care about how the kernel identifies whether a page is
dirty. It could be dirtied due to pinning, or due to IOMMU dirty bit, or due
to IOMMU page fault. Here we'd better just focus on user-tangible effect,
e.g. a large/non-converging dirty map might be returned then how to handle
such situation...

> +
> +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 the vendor driver using
> +vfio_pin_pages() will be copied to destination in both the phases. Copying
> +dirty pages in pre-copy phase helps QEMU to predict if it can achieve its
> +downtime tolerances.

worthy of some elaboration on the last sentence.

> +
> +QEMU also provides a per device opt-out option ``pre-copy-dirty-page-
> tracking``
> +to disable dirty page tracking during pre-copy phase. If it is set to off, 
> all

IIUC dirty page tracking is always enabled in vfio_save_setup. What this option
does is to skip sync-ing dirty bitmap in vfio_listerner_log_sync.

> +pinned pages will be copied to destination in stop-and-copy phase only.
> +
> +System memory dirty pages tracking when vIOMMU is enabled
> +---------------------------------------------------------
> +
> +With vIOMMU, an IO virtual address range can get unmapped while in pre-
> copy
> +phase of migration. In that case, the unmap ioctl returns any pinned pages
> in
> +that range and QEMU reports corresponding guest physical pages dirty.

pinned pages -> dirty pages

> During
> +stop-and-copy phase, an IOMMU notifier is used to get a callback for
> mapped
> +pages and then dirty pages bitmap is fetched from VFIO IOMMU modules
> for those
> +mapped ranges.
> +
> +Flow of state changes during Live migration
> +===========================================
> +
> +Below is the flow of state change during live migration.
> +The values in the brackets represent the VM state, the migration state, and
> +the VFIO device state, respectively.
> +
> +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()
> +                    (RUNNING, _SETUP, _RUNNING|_SAVING)
> +                                  |
> +                    (RUNNING, _ACTIVE, _RUNNING|_SAVING)
> +             If device is active, get pending_bytes by .save_live_pending()
> +          If total pending_bytes >= threshold_size, call .save_live_iterate()
> +                  Data of VFIO device for pre-copy phase is copied
> +        Iterate till total pending bytes converge and are less than threshold
> +                                  |
> +  On migration completion, vCPU stops and
> calls .save_live_complete_precopy for
> +   each active device. The VFIO device is then transitioned into _SAVING
> state
> +                   (FINISH_MIGRATE, _DEVICE, _SAVING)
> +                                  |
> +     For the VFIO device, iterate in .save_live_complete_precopy until
> +                         pending data is 0
> +                   (FINISH_MIGRATE, _DEVICE, _STOPPED)
> +                                  |
> +                 (FINISH_MIGRATE, _COMPLETED, _STOPPED)
> +             Migraton thread schedules cleanup bottom half and exits
> +
> +Live migration resume path
> +--------------------------
> +
> +::
> +
> +              Incoming migration calls .load_setup for each device
> +                       (RESTORE_VM, _ACTIVE, _STOPPED)
> +                                 |
> +       For each device, .load_state is called for that device section data
> +                       (RESTORE_VM, _ACTIVE, _RESUMING)
> +                                 |
> +    At the end, .load_cleanup is called for each device and vCPUs are started
> +                       (RUNNING, _NONE, _RUNNING)
> +
> +Postcopy
> +========
> +
> +Postcopy migration is not supported for VFIO devices.
> --
> 2.27.0

Thanks
Kevin