[PATCH v5 0/4] migration: UFFD write-tracking migration/snapshots

[Andrey Gruzdev via <qemu-devel@nongnu.org>]

This patch series is a kind of 'rethinking' of Denis Plotnikov's ideas he's
implemented in his series '[PATCH v0 0/4] migration: add background snapshot'.

Currently the only way to make (external) live VM snapshot is using existing
dirty page logging migration mechanism. The main problem is that it tends to
produce a lot of page duplicates while running VM goes on updating already
saved pages. That leads to the fact that vmstate image size is commonly several
times bigger then non-zero part of virtual machine's RSS. Time required to
converge RAM migration and the size of snapshot image severely depend on the
guest memory write rate, sometimes resulting in unacceptably long snapshot
creation time and huge image size.

This series propose a way to solve the aforementioned problems. This is done
by using different RAM migration mechanism based on UFFD write protection
management introduced in v5.7 kernel. The migration strategy is to 'freeze'
guest RAM content using write-protection and iteratively release protection
for memory ranges that have already been saved to the migration stream.
At the same time we read in pending UFFD write fault events and save those
pages out-of-order with higher priority.

How to use:
1. Enable write-tracking migration capability
   virsh qemu-monitor-command <domain> --hmp migrate_set_capability.
track-writes-ram on

2. Start the external migration to a file
   virsh qemu-monitor-command <domain> --hmp migrate exec:'cat > ./vm_state'

3. Wait for the migration finish and check that the migration has completed.
state.

Changes v4->v5:

* 1. Refactored util/userfaultfd.c code to support features required by postcopy.
* 2. Introduced checks for host kernel and guest memory backend compatibility
*    to 'background-snapshot' branch in migrate_caps_check().
* 3. Switched to using trace_xxx instead of info_report()/error_report() for
*    cases when error message must be hidden (probing UFFD-IO) or info may be
*    really littering output if goes to stderr.
* 4  Added RCU_READ_LOCK_GUARDs to the code dealing with RAM block list.
* 5. Added memory_region_ref() for each RAM block being wr-protected.
* 6. Reused qemu_ram_block_from_host() instead of custom RAM block lookup routine.
* 7. Refused from using specific hwaddr/ram_addr_t in favour of void */uint64_t.
* 8. Currently dropped 'linear-scan-rate-limiting' patch. The reason is that
*    that choosen criteria for high-latency fault detection (i.e. timestamp of
*    UFFD event fetch) is not representative enough for this task.
*    At the moment it looks somehow like premature optimization effort.
* 8. Dropped some unnecessary/unused code.

Andrey Gruzdev (4):
  migration: introduce 'background-snapshot' migration capability
  migration: introduce UFFD-WP low-level interface helpers
  migration: support UFFD write fault processing in ram_save_iterate()
  migration: implementation of background snapshot thread

 include/exec/memory.h      |   8 +
 include/qemu/userfaultfd.h |  35 ++++
 migration/migration.c      | 363 ++++++++++++++++++++++++++++++++++++-
 migration/migration.h      |   4 +
 migration/ram.c            | 270 +++++++++++++++++++++++++++
 migration/ram.h            |   6 +
 migration/savevm.c         |   1 -
 migration/savevm.h         |   2 +
 migration/trace-events     |   2 +
 qapi/migration.json        |   7 +-
 util/meson.build           |   1 +
 util/trace-events          |   9 +
 util/userfaultfd.c         | 345 +++++++++++++++++++++++++++++++++++
 13 files changed, 1049 insertions(+), 4 deletions(-)
 create mode 100644 include/qemu/userfaultfd.h
 create mode 100644 util/userfaultfd.c

-- 
2.25.1