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From: "Cédric Le Goater" <clg@redhat.com>
To: "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>,
	Peter Xu <peterx@redhat.com>, Fabiano Rosas <farosas@suse.de>
Cc: "Alex Williamson" <alex.williamson@redhat.com>,
	"Eric Blake" <eblake@redhat.com>,
	"Markus Armbruster" <armbru@redhat.com>,
	"Daniel P . Berrangé" <berrange@redhat.com>,
	"Avihai Horon" <avihaih@nvidia.com>,
	"Joao Martins" <joao.m.martins@oracle.com>,
	qemu-devel@nongnu.org
Subject: Re: [PATCH v5 36/36] vfio/migration: Update VFIO migration documentation
Date: Thu, 27 Feb 2025 07:59:45 +0100	[thread overview]
Message-ID: <cbebee68-f891-4946-885d-01a8ed24e6c1@redhat.com> (raw)
In-Reply-To: <2031790b755aa95b61470b286fa787b78be08107.1739994627.git.maciej.szmigiero@oracle.com>

On 2/19/25 21:34, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> 
> Update the VFIO documentation at docs/devel/migration describing the
> changes brought by the multifd device state transfer.
> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> ---
>   docs/devel/migration/vfio.rst | 80 +++++++++++++++++++++++++++++++----
>   1 file changed, 71 insertions(+), 9 deletions(-)
> 
> diff --git a/docs/devel/migration/vfio.rst b/docs/devel/migration/vfio.rst
> index c49482eab66d..d9b169d29921 100644
> --- a/docs/devel/migration/vfio.rst
> +++ b/docs/devel/migration/vfio.rst
> @@ -16,6 +16,37 @@ helps to reduce the total downtime of the VM. VFIO devices opt-in to pre-copy
>   support by reporting the VFIO_MIGRATION_PRE_COPY flag in the
>   VFIO_DEVICE_FEATURE_MIGRATION ioctl.

Please add a new "multifd" documentation subsection at the end of the file
with this part :

> +Starting from QEMU version 10.0 there's a possibility to transfer VFIO device
> +_STOP_COPY state via multifd channels. This helps reduce downtime - especially
> +with multiple VFIO devices or with devices having a large migration state.
> +As an additional benefit, setting the VFIO device to _STOP_COPY state and
> +saving its config space is also parallelized (run in a separate thread) in
> +such migration mode.
> +
> +The multifd VFIO device state transfer is controlled by
> +"x-migration-multifd-transfer" VFIO device property. This property defaults to
> +AUTO, which means that VFIO device state transfer via multifd channels is
> +attempted in configurations that otherwise support it.
> +

I was expecting a much more detailed explanation on the design too  :

  * in the cover letter
  * in the hw/vfio/migration-multifd.c
  * in some new file under docs/devel/migration/



This section :

> +Since the target QEMU needs to load device state buffers in-order it needs to
> +queue incoming buffers until they can be loaded into the device.
> +This means that a malicious QEMU source could theoretically cause the target
> +QEMU to allocate unlimited amounts of memory for such buffers-in-flight.
> +
> +The "x-migration-max-queued-buffers" property allows capping the maximum count
> +of these VFIO device state buffers queued at the destination.
> +
> +Because a malicious QEMU source causing OOM on the target is not expected to be
> +a realistic threat in most of VFIO live migration use cases and the right value
> +depends on the particular setup by default this queued buffers limit is
> +disabled by setting it to UINT64_MAX.

should be in patch 34. It is not obvious it will be merged.


This section :

> +Some host platforms (like ARM64) require that VFIO device config is loaded only
> +after all iterables were loaded.
> +Such interlocking is controlled by "x-migration-load-config-after-iter" VFIO
> +device property, which in its default setting (AUTO) does so only on platforms
> +that actually require it.

Should be in 35. Same reason.


>   When pre-copy is supported, it's possible to further reduce downtime by
>   enabling "switchover-ack" migration capability.
>   VFIO migration uAPI defines "initial bytes" as part of its pre-copy data stream
> @@ -67,14 +98,39 @@ VFIO implements the device hooks for the iterative approach as follows:
>   * A ``switchover_ack_needed`` function that checks if the VFIO device uses
>     "switchover-ack" migration capability when this capability is enabled.
>   
> -* A ``save_state`` function to save the device config space if it is present.
> -
> -* A ``save_live_complete_precopy`` function that sets the VFIO device in
> -  _STOP_COPY state and iteratively copies the data for the VFIO device until
> -  the vendor driver indicates that no data remains.
> -
> -* A ``load_state`` function that loads the config section and the data
> -  sections that are generated by the save functions above.
> +* A ``switchover_start`` function that in the multifd mode starts a thread that
> +  reassembles the multifd received data and loads it in-order into the device.
> +  In the non-multifd mode this function is a NOP.
> +
> +* A ``save_state`` function to save the device config space if it is present
> +  in the non-multifd mode.
> +  In the multifd mode it just emits either a dummy EOS marker or
> +  "all iterables were loaded" flag for configurations that need to defer
> +  loading device config space after them.
> +
> +* A ``save_live_complete_precopy`` function that in the non-multifd mode sets
> +  the VFIO device in _STOP_COPY state and iteratively copies the data for the
> +  VFIO device until the vendor driver indicates that no data remains.
> +  In the multifd mode it just emits a dummy EOS marker.
> +
> +* A ``save_live_complete_precopy_thread`` function that in the multifd mode
> +  provides thread handler performing multifd device state transfer.
> +  It sets the VFIO device to _STOP_COPY state, iteratively reads the data
> +  from the VFIO device and queues it for multifd transmission until the vendor
> +  driver indicates that no data remains.
> +  After that, it saves the device config space and queues it for multifd
> +  transfer too.
> +  In the non-multifd mode this thread is a NOP.
> +
> +* A ``load_state`` function that loads the data sections that are generated
> +  by the main migration channel save functions above.
> +  In the non-multifd mode it also loads the config section, while in the
> +  multifd mode it handles the optional "all iterables were loaded" flag if
> +  it is in use.
> +
> +* A ``load_state_buffer`` function that loads the device state and the device
> +  config that arrived via multifd channels.
> +  It's used only in the multifd mode.

Please move the documentation of the new migration handlers in the
patch introducing them.


Thanks,

C.



>   
>   * ``cleanup`` functions for both save and load that perform any migration
>     related cleanup.
> @@ -176,8 +232,11 @@ Live migration save path
>                   Then the VFIO device is put in _STOP_COPY state
>                        (FINISH_MIGRATE, _ACTIVE, _STOP_COPY)
>            .save_live_complete_precopy() is called for each active device
> -      For the VFIO device, iterate in .save_live_complete_precopy() until
> +              For the VFIO device: in the non-multifd mode iterate in
> +                        .save_live_complete_precopy() until
>                                  pending data is 0
> +	          In the multifd mode this iteration is done in
> +	          .save_live_complete_precopy_thread() instead.
>                                         |
>                        (POSTMIGRATE, _COMPLETED, _STOP_COPY)
>               Migraton thread schedules cleanup bottom half and exits
> @@ -194,6 +253,9 @@ Live migration resume path
>                             (RESTORE_VM, _ACTIVE, _STOP)
>                                         |
>        For each device, .load_state() is called for that device section data
> +                 transmitted via the main migration channel.
> +     For data transmitted via multifd channels .load_state_buffer() is called
> +                                   instead.
>                           (RESTORE_VM, _ACTIVE, _RESUMING)
>                                         |
>     At the end, .load_cleanup() is called for each device and vCPUs are started
> 



  reply	other threads:[~2025-02-27  7:00 UTC|newest]

Thread overview: 120+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2025-02-19 20:33 [PATCH v5 00/36] Multifd 🔀 device state transfer support with VFIO consumer Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 01/36] migration: Clarify that {load, save}_cleanup handlers can run without setup Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 02/36] thread-pool: Remove thread_pool_submit() function Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 03/36] thread-pool: Rename AIO pool functions to *_aio() and data types to *Aio Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 04/36] thread-pool: Implement generic (non-AIO) pool support Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 05/36] migration: Add MIG_CMD_SWITCHOVER_START and its load handler Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 06/36] migration: Add qemu_loadvm_load_state_buffer() and its handler Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 07/36] migration: postcopy_ram_listen_thread() should take BQL for some calls Maciej S. Szmigiero
2025-02-25 17:16   ` Peter Xu
2025-02-25 21:08     ` Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 08/36] error: define g_autoptr() cleanup function for the Error type Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 09/36] migration: Add thread pool of optional load threads Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 10/36] migration/multifd: Split packet into header and RAM data Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 11/36] migration/multifd: Device state transfer support - receive side Maciej S. Szmigiero
2025-03-02 12:42   ` Avihai Horon
2025-03-03 22:14     ` Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 12/36] migration/multifd: Make multifd_send() thread safe Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 13/36] migration/multifd: Add an explicit MultiFDSendData destructor Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 14/36] migration/multifd: Device state transfer support - send side Maciej S. Szmigiero
2025-03-02 12:46   ` Avihai Horon
2025-03-03 22:15     ` Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 15/36] migration/multifd: Make MultiFDSendData a struct Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 16/36] migration/multifd: Add multifd_device_state_supported() Maciej S. Szmigiero
2025-02-19 20:33 ` [PATCH v5 17/36] migration: Add save_live_complete_precopy_thread handler Maciej S. Szmigiero
2025-02-26 16:43   ` Peter Xu
2025-03-04 21:50     ` Maciej S. Szmigiero
2025-03-04 22:03       ` Peter Xu
2025-02-19 20:34 ` [PATCH v5 18/36] vfio/migration: Add load_device_config_state_start trace event Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 19/36] vfio/migration: Convert bytes_transferred counter to atomic Maciej S. Szmigiero
2025-02-26  7:52   ` Cédric Le Goater
2025-02-26 13:55     ` Maciej S. Szmigiero
2025-02-26 15:56       ` Cédric Le Goater
2025-02-26 16:20   ` Cédric Le Goater
2025-02-19 20:34 ` [PATCH v5 20/36] vfio/migration: Add vfio_add_bytes_transferred() Maciej S. Szmigiero
2025-02-26  8:06   ` Cédric Le Goater
2025-02-26 15:45     ` Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 21/36] vfio/migration: Move migration channel flags to vfio-common.h header file Maciej S. Szmigiero
2025-02-26  8:19   ` Cédric Le Goater
2025-02-19 20:34 ` [PATCH v5 22/36] vfio/migration: Multifd device state transfer support - basic types Maciej S. Szmigiero
2025-02-26  8:52   ` Cédric Le Goater
2025-02-26 16:06     ` Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 23/36] vfio/migration: Multifd device state transfer support - VFIOStateBuffer(s) Maciej S. Szmigiero
2025-02-26  8:54   ` Cédric Le Goater
2025-03-02 13:00   ` Avihai Horon
2025-03-02 15:14     ` Maciej S. Szmigiero
2025-03-03  6:42     ` Cédric Le Goater
2025-03-03 22:14       ` Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 24/36] vfio/migration: Multifd device state transfer - add support checking function Maciej S. Szmigiero
2025-02-26  8:54   ` Cédric Le Goater
2025-02-19 20:34 ` [PATCH v5 25/36] vfio/migration: Multifd device state transfer support - receive init/cleanup Maciej S. Szmigiero
2025-02-26 10:14   ` Cédric Le Goater
2025-02-26 17:22     ` Cédric Le Goater
2025-02-26 17:28       ` Maciej S. Szmigiero
2025-02-26 17:28   ` Cédric Le Goater
2025-02-27 22:00     ` Maciej S. Szmigiero
2025-02-26 17:46   ` Cédric Le Goater
2025-02-27 22:00     ` Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 26/36] vfio/migration: Multifd device state transfer support - received buffers queuing Maciej S. Szmigiero
2025-02-26 10:43   ` Cédric Le Goater
2025-02-26 21:04     ` Maciej S. Szmigiero
2025-02-28  8:09       ` Cédric Le Goater
2025-02-28 20:47         ` Maciej S. Szmigiero
2025-03-02 13:12   ` Avihai Horon
2025-03-03 22:15     ` Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 27/36] vfio/migration: Multifd device state transfer support - load thread Maciej S. Szmigiero
2025-02-26 13:49   ` Cédric Le Goater
2025-02-26 21:05     ` Maciej S. Szmigiero
2025-02-28  9:11       ` Cédric Le Goater
2025-02-28 20:48         ` Maciej S. Szmigiero
2025-03-02 14:19     ` Avihai Horon
2025-03-03 22:16       ` Maciej S. Szmigiero
2025-03-02 14:15   ` Avihai Horon
2025-03-03 22:16     ` Maciej S. Szmigiero
2025-03-04 11:21       ` Avihai Horon
2025-02-19 20:34 ` [PATCH v5 28/36] vfio/migration: Multifd device state transfer support - config loading support Maciej S. Szmigiero
2025-02-26 13:52   ` Cédric Le Goater
2025-02-26 21:05     ` Maciej S. Szmigiero
2025-03-02 14:25   ` Avihai Horon
2025-03-03 22:17     ` Maciej S. Szmigiero
2025-03-04  7:41       ` Cédric Le Goater
2025-03-04 21:50         ` Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 29/36] migration/qemu-file: Define g_autoptr() cleanup function for QEMUFile Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 30/36] vfio/migration: Multifd device state transfer support - send side Maciej S. Szmigiero
2025-02-26 16:43   ` Cédric Le Goater
2025-02-26 21:05     ` Maciej S. Szmigiero
2025-02-28  9:13       ` Cédric Le Goater
2025-02-28 20:49         ` Maciej S. Szmigiero
2025-03-02 14:41   ` Avihai Horon
2025-03-03 22:17     ` Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 31/36] vfio/migration: Add x-migration-multifd-transfer VFIO property Maciej S. Szmigiero
2025-02-27  6:45   ` Cédric Le Goater
2025-03-02 14:48   ` Avihai Horon
2025-03-03 22:17     ` Maciej S. Szmigiero
2025-03-04 11:29       ` Avihai Horon
2025-03-04 21:50         ` Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 32/36] vfio/migration: Make x-migration-multifd-transfer VFIO property mutable Maciej S. Szmigiero
2025-02-26 17:59   ` Cédric Le Goater
2025-02-26 21:05     ` Maciej S. Szmigiero
2025-02-28  8:44       ` Cédric Le Goater
2025-02-28 20:47         ` Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 33/36] hw/core/machine: Add compat for x-migration-multifd-transfer VFIO property Maciej S. Szmigiero
2025-02-26 17:59   ` Cédric Le Goater
2025-02-19 20:34 ` [PATCH v5 34/36] vfio/migration: Max in-flight VFIO device state buffer count limit Maciej S. Szmigiero
2025-02-27  6:48   ` Cédric Le Goater
2025-02-27 22:01     ` Maciej S. Szmigiero
2025-02-28  8:53       ` Cédric Le Goater
2025-02-28 20:48         ` Maciej S. Szmigiero
2025-03-02 14:53   ` Avihai Horon
2025-03-02 14:54     ` Maciej S. Szmigiero
2025-03-02 14:59       ` Maciej S. Szmigiero
2025-03-02 16:28         ` Avihai Horon
2025-02-19 20:34 ` [PATCH v5 35/36] vfio/migration: Add x-migration-load-config-after-iter VFIO property Maciej S. Szmigiero
2025-02-19 20:34 ` [PATCH v5 36/36] vfio/migration: Update VFIO migration documentation Maciej S. Szmigiero
2025-02-27  6:59   ` Cédric Le Goater [this message]
2025-02-27 22:01     ` Maciej S. Szmigiero
2025-02-28 10:05       ` Cédric Le Goater
2025-02-28 20:49         ` Maciej S. Szmigiero
2025-02-28 23:38         ` Fabiano Rosas
2025-03-03  9:34           ` Cédric Le Goater
2025-03-03 22:14           ` Maciej S. Szmigiero

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