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

[Shenming Lu <lushenming@huawei.com>]

On 2021/3/26 21:18, Tarun Gupta 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.
> 
> 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.
> 
> 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 | 143 ++++++++++++++++++++++++++++++++++
>  3 files changed, 145 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..24cb55991a
> --- /dev/null
> +++ b/docs/devel/vfio-migration.rst
> @@ -0,0 +1,143 @@
> +=====================
> +VFIO device Migration
> +=====================
> +
> +Migration of virtual machine involves saving the state for each device that
> +the guest is running on source host and restoring this saved state on the
> +destination host. This document details how saving and restoring of VFIO
> +devices is done in QEMU.
> +
> +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 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
> +  copies the remaining data for the VFIO device until the vendor driver
> +  indicates that no data remains (pending bytes is zero).

Hi Tarun,

We have moved the saving of the config space to the ``save_state`` function
added in commit d329f5032e1, do we need to add this change here? :-)

Thanks,
Shenming

> +
> +* 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 the
> +corresponding VFIO device state and conveyed to the 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 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
> +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
> +destination in both the phases. Copying dirty pages in pre-copy phase helps
> +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``
> +which disables querying dirty bitmap during pre-copy phase. If it is set to
> +off, all dirty 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 dirty pages in
> +that range and QEMU reports corresponding guest physical pages dirty. 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 currently not supported for VFIO devices.
>