Re: [PATCH v4 00/33] Multifd 🔀 device state transfer support with VFIO consumer

[Maciej S. Szmigiero]

On 30.01.2025 21:19, Fabiano Rosas wrote:
> "Maciej S. Szmigiero" <mail@maciej.szmigiero.name> writes:
>
> > From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> >
> > This is an updated v4 patch series of the v3 series located here:
> > https://lore.kernel.org/qemu-devel/cover.1731773021.git.maciej.szmigiero@oracle.com/
> >
> > Changes from v3:
> > * MigrationLoadThread now returns bool and an Error complex error type
> > instead of just an int.
> >
> > * qemu_loadvm_load_thread_pool now reports error via migrate_set_error()
> > instead of dedicated load_threads_ret variable.
> >
> > * Since the change above uncovered an issue with respect to multifd send
> > channels not terminating TLS session properly QIOChannelTLS now allows
> > gracefully handling this situation.
> >
> > * qemu_loadvm_load_thread_pool state is now part of MigrationIncomingState
> > instead of being stored in global variables.
> > This state now also has its own init/cleanup helpers.
> >
> > * qemu_loadvm_load_thread_pool code is now moved into a separate section
> > of the savevm.c file, marked by an appropriate comment.
> >
> > * thread_pool_free() is now documented to have wait-before-free semantic,
> > which allowed removal of explicit waits from thread pool cleanup paths.
> >
> > * thread_pool_submit_immediate() method was added since this functionality
> > is used by both generic thread pool users in this patch set.
> >
> > * postcopy_ram_listen_thread() now takes BQL around function calls that
> > ultimately call migration methods requiring BQL.
> > This fixes one of QEMU tests failing when explicitly BQL-sensitive code
> > is added later to these methods.
> >
> > * qemu_loadvm_load_state_buffer() now returns a bool value instead of int.
> >
> > * "Send final SYNC only after device state is complete" patch was
> > dropped since Peter implemented equivalent functionality upstream.
> >
> > * "Document the BQL behavior of load SaveVMHandlers" patch was dropped
> > since that's something better done later, separately from this patch set.
> >
> > * Header size is now added to mig_stats.multifd_bytes where it is actually
> > sent in the zero copy case - in multifd_nocomp_send_prepare().
> >
> > * Spurious wakeups from qemu_cond_wait() are now handled properly as
> > pointed out by Avihai.
> >
> > * VFIO migration FD now allows partial write() completion as pointed out
> > by Avihai.
> >
> > * Patch "vfio/migration: Don't run load cleanup if load setup didn't run"
> > was dropped, instead all objects related to multifd load are now located in
> > their own VFIOMultifd struct which is allocated only if multifd device state
> > transfer is actually in use.
> >
> > * Intermediate VFIOStateBuffers API as suggested by Avihai is now introduced
> > to simplify vfio_load_state_buffer() and vfio_load_bufs_thread().
> >
> > * Optional VFIO device config state loading interlocking with loading
> > other iterables is now possible due to ARM64 platform VFIO dependency on
> > interrupt controller being loaded first as pointed out by Avihai.
> >
> > * Patch "Multifd device state transfer support - receive side" was split
> > into a few smaller patches as suggested by Cédric.
> >
> > * x-migration-multifd-transfer VFIO property compat changes were moved
> > into a separate patch as suggested by Cédric.
> >
> > * Other small changes, like renamed functions and variables/members, added
> > review tags, code formatting, moved QEMU_LOCK_GUARD() instances closer to
> > actual protected blocks, etc.
> >
> > ========================================================================
> >
> > This patch set is targeting QEMU 10.0.
> >
> > What's not yet present is documentation update under docs/devel/migration
> > but I didn't want to delay posting the code any longer.
> > Such doc can still be merged later when the design is 100% finalized.
> >
> > ========================================================================
> >
> > Maciej S. Szmigiero (32):
> >    migration: Clarify that {load,save}_cleanup handlers can run without
> >      setup
> >    thread-pool: Remove thread_pool_submit() function
> >    thread-pool: Rename AIO pool functions to *_aio() and data types to
> >      *Aio
> >    thread-pool: Implement generic (non-AIO) pool support
> >    migration: Add MIG_CMD_SWITCHOVER_START and its load handler
> >    migration: Add qemu_loadvm_load_state_buffer() and its handler
> >    io: tls: Allow terminating the TLS session gracefully with EOF
> >    migration/multifd: Allow premature EOF on TLS incoming channels
> >    migration: postcopy_ram_listen_thread() needs to take BQL for some
> >      calls
> >    error: define g_autoptr() cleanup function for the Error type
> >    migration: Add thread pool of optional load threads
> >    migration/multifd: Split packet into header and RAM data
> >    migration/multifd: Device state transfer support - receive side
> >    migration/multifd: Make multifd_send() thread safe
> >    migration/multifd: Add an explicit MultiFDSendData destructor
> >    migration/multifd: Device state transfer support - send side
> >    migration/multifd: Add multifd_device_state_supported()
> >    migration: Add save_live_complete_precopy_thread handler
> >    vfio/migration: Add x-migration-load-config-after-iter VFIO property
> >    vfio/migration: Add load_device_config_state_start trace event
> >    vfio/migration: Convert bytes_transferred counter to atomic
> >    vfio/migration: Multifd device state transfer support - basic types
> >    vfio/migration: Multifd device state transfer support -
> >      VFIOStateBuffer(s)
> >    vfio/migration: Multifd device state transfer - add support checking
> >      function
> >    vfio/migration: Multifd device state transfer support - receive
> >      init/cleanup
> >    vfio/migration: Multifd device state transfer support - received
> >      buffers queuing
> >    vfio/migration: Multifd device state transfer support - load thread
> >    vfio/migration: Multifd device state transfer support - config loading
> >      support
> >    migration/qemu-file: Define g_autoptr() cleanup function for QEMUFile
> >    vfio/migration: Multifd device state transfer support - send side
> >    vfio/migration: Add x-migration-multifd-transfer VFIO property
> >    hw/core/machine: Add compat for x-migration-multifd-transfer VFIO
> >      property
> >
> > Peter Xu (1):
> >    migration/multifd: Make MultiFDSendData a struct
> >
> >   hw/core/machine.c                  |   2 +
> >   hw/vfio/migration.c                | 754 ++++++++++++++++++++++++++++-
> >   hw/vfio/pci.c                      |  14 +
> >   hw/vfio/trace-events               |  11 +-
> >   include/block/aio.h                |   8 +-
> >   include/block/thread-pool.h        |  62 ++-
> >   include/hw/vfio/vfio-common.h      |   7 +
> >   include/io/channel-tls.h           |  11 +
> >   include/migration/client-options.h |   4 +
> >   include/migration/misc.h           |  16 +
> >   include/migration/register.h       |  54 ++-
> >   include/qapi/error.h               |   2 +
> >   include/qemu/typedefs.h            |   6 +
> >   io/channel-tls.c                   |   6 +
> >   migration/colo.c                   |   3 +
> >   migration/meson.build              |   1 +
> >   migration/migration-hmp-cmds.c     |   2 +
> >   migration/migration.c              |   6 +-
> >   migration/migration.h              |   7 +
> >   migration/multifd-device-state.c   | 192 ++++++++
> >   migration/multifd-nocomp.c         |  30 +-
> >   migration/multifd.c                | 248 ++++++++--
> >   migration/multifd.h                |  74 ++-
> >   migration/options.c                |   9 +
> >   migration/qemu-file.h              |   2 +
> >   migration/savevm.c                 | 195 +++++++-
> >   migration/savevm.h                 |   6 +-
> >   migration/trace-events             |   1 +
> >   scripts/analyze-migration.py       |  11 +
> >   tests/unit/test-thread-pool.c      |   6 +-
> >   util/async.c                       |   6 +-
> >   util/thread-pool.c                 | 184 +++++--
> >   util/trace-events                  |   6 +-
> >   33 files changed, 1814 insertions(+), 132 deletions(-)
> >   create mode 100644 migration/multifd-device-state.c
>
> Hi!
>
> We have build issues:
>
> https://gitlab.com/farosas/qemu/-/pipelines/1649146958

Looks like that's an issue that qatomics on 64-bit
VFIO bytes transferred counters aren't available on
32-bit host platforms.

The easiest way would be probably to change these to
32-bit counters on 32-bit platforms since they can't
realistically address more memory anyway.

> And the postcopy/recovery test is failing. It seems the migration
> finishes before the test can issue migrate-pause:
>
> QTEST_QEMU_BINARY=./qemu-system-x86_64  ./tests/qtest/migration-test -p
> /x86_64/migration/postcopy/recovery/plain
> ...
> {"execute": "migrate-start-postcopy"}
> {"return": {}}
> {"secs": 1738267018, "usecs": 860991}, "event": "MIGRATION", "data": {"status": 
> "postcopy-active"}
> {"secs": 1738267018, "usecs": 861284}, "event": "STOP"
> {"secs": 1738267017, "usecs": 960322}, "event": "MIGRATION", "data": {"status": 
> "active"}
> {"secs": 1738267018, "usecs": 865589}, "event": "MIGRATION", "data": {"status": 
> "postcopy-active"}
> {"secs": 1738267099, "usecs": 120971}, "event": "MIGRATION", "data": {"status": 
> "completed"}
> {"secs": 1738267099, "usecs": 121154}, "event": "RESUME"
> {"execute": "query-migrate"}
>
> ERROR:../tests/qtest/migration/migration-qmp.c:172:check_migration_status:
> assertion failed (current_status != "completed"): ("completed" !=
> "completed")

Hmm, it looks like this failure wasn't showing
in my tests because the test was skipped due to
missing userfaultfd support:

$ QTEST_QEMU_BINARY=./qemu-system-x86_64 ./tests/qtest/migration-test -p 
/x86_64/migration/postcopy/recovery/plain
TAP version 14
# random seed: R02Sc99a7d93274064bb87f3e0789fbf8326
# Skipping test: userfaultfd not available
# Start of x86_64 tests
# Start of migration tests
# End of migration tests
# End of x86_64 tests
1..0

Will try to make this test run and investigate the reason for
failure.

Thanks,
Maciej