[Qemu-devel] [PATCH 00/21][RFC] postcopy live migration

[Isaku Yamahata]

Intro
=====
This patch series implements postcopy live migration.[1]
As discussed at KVM forum 2011, dedicated character device is used for
distributed shared memory between migration source and destination.
Now we can discuss/benchmark/compare with precopy. I believe there are
much rooms for improvement.

[1] http://wiki.qemu.org/Features/PostCopyLiveMigration


Usage
=====
You need load umem character device on the host before starting migration.
Postcopy can be used for tcg and kvm accelarator. The implementation depend
on only linux umem character device. But the driver dependent code is split
into a file.
I tested only host page size == guest page size case, but the implementation
allows host page size != guest page size case.

The following options are added with this patch series.
- incoming part
  command line options
  -postcopy [-postcopy-flags <flags>]
  where flags is for changing behavior for benchmark/debugging
  Currently the following flags are available
  0: default
  1: enable touching page request

  example:
  qemu -postcopy -incoming tcp:0:4444 -monitor stdio -machine accel=kvm

- outging part
  options for migrate command 
  migrate [-p [-n]] URI
  -p: indicate postcopy migration
  -n: disable background transferring pages: This is for benchmark/debugging

  example:
  migrate -p -n tcp:<dest ip address>:4444


TODO
====
- benchmark/evaluation. Especially how async page fault affects the result.
- improve/optimization
  At the moment at least what I'm aware of is
  - touching pages in incoming qemu process by fd handler seems suboptimal.
    creating dedicated thread?
  - making incoming socket non-blocking
  - outgoing handler seems suboptimal causing latency.
- catch up memory API change
- consider on FUSE/CUSE possibility
- and more...

basic postcopy work flow
========================
        qemu on the destination
              |
              V
        open(/dev/umem)
              |
              V
        UMEM_DEV_CREATE_UMEM
              |
              V
        Here we have two file descriptors to
        umem device and shmem file
              |
              |                                  umemd
              |                                  daemon on the destination
              |
              V    create pipe to communicate
        fork()---------------------------------------,
              |                                      |
              V                                      |
        close(socket)                                V
        close(shmem)                              mmap(shmem file)
              |                                      |
              V                                      V
        mmap(umem device) for guest RAM           close(shmem file)
              |                                      |
        close(umem device)                           |
              |                                      |
              V                                      |
        wait for ready from daemon <----pipe-----send ready message
              |                                      |
              |                                 Here the daemon takes over 
        send ok------------pipe---------------> the owner of the socket    
              |                                 to the source              
              V                                      |
        entering post copy stage                     |
        start guest execution                        |
              |                                      |
              V                                      V
        access guest RAM                          UMEM_GET_PAGE_REQUEST
              |                                      |
              V                                      V
        page fault ------------------------------>page offset is returned
        block                                        |
                                                     V
                                                  pull page from the source
                                                  write the page contents
                                                  to the shmem.
                                                     |
                                                     V
        unblock     <-----------------------------UMEM_MARK_PAGE_CACHED
        the fault handler returns the page
        page fault is resolved
              |
              |                                   pages can be sent
              |                                   backgroundly
              |                                      |
              |                                      V
              |                                   UMEM_MARK_PAGE_CACHED
              |                                      |
              V                                      V
        The specified pages<-----pipe------------request to touch pages
        are made present by                          |
        touching guest RAM.                          |
              |                                      |
              V                                      V
             reply-------------pipe-------------> release the cached page
              |                                   madvise(MADV_REMOVE)
              |                                      |
              V                                      V

                 all the pages are pulled from the source

              |                                      |
              V                                      V
        the vma becomes anonymous<----------------UMEM_MAKE_VMA_ANONYMOUS
       (note: I'm not sure if this can be implemented or not)
              |                                      |
              V                                      V
        migration completes                        exit()



Isaku Yamahata (21):
  arch_init: export sort_ram_list() and ram_save_block()
  arch_init: export RAM_SAVE_xxx flags for postcopy
  arch_init/ram_save: introduce constant for ram save version = 4
  arch_init: refactor host_from_stream_offset()
  arch_init/ram_save_live: factor out RAM_SAVE_FLAG_MEM_SIZE case
  arch_init: refactor ram_save_block()
  arch_init/ram_save_live: factor out ram_save_limit
  arch_init/ram_load: refactor ram_load
  exec.c: factor out qemu_get_ram_ptr()
  exec.c: export last_ram_offset()
  savevm: export qemu_peek_buffer, qemu_peek_byte, qemu_file_skip
  savevm: qemu_pending_size() to return pending buffered size
  savevm, buffered_file: introduce method to drain buffer of buffered
    file
  migration: export migrate_fd_completed() and migrate_fd_cleanup()
  migration: factor out parameters into MigrationParams
  umem.h: import Linux umem.h
  update-linux-headers.sh: teach umem.h to update-linux-headers.sh
  configure: add CONFIG_POSTCOPY option
  postcopy: introduce -postcopy and -postcopy-flags option
  postcopy outgoing: add -p and -n option to migrate command
  postcopy: implement postcopy livemigration

 Makefile.target                 |    4 +
 arch_init.c                     |  260 ++++---
 arch_init.h                     |   20 +
 block-migration.c               |    8 +-
 buffered_file.c                 |   20 +-
 buffered_file.h                 |    1 +
 configure                       |   12 +
 cpu-all.h                       |    9 +
 exec-obsolete.h                 |    1 +
 exec.c                          |   75 +-
 hmp-commands.hx                 |   12 +-
 hw/hw.h                         |    7 +-
 linux-headers/linux/umem.h      |   83 ++
 migration-exec.c                |    8 +
 migration-fd.c                  |   30 +
 migration-postcopy-stub.c       |   77 ++
 migration-postcopy.c            | 1891 +++++++++++++++++++++++++++++++++++++++
 migration-tcp.c                 |   37 +-
 migration-unix.c                |   32 +-
 migration.c                     |   53 +-
 migration.h                     |   49 +-
 qemu-common.h                   |    2 +
 qemu-options.hx                 |   25 +
 qmp-commands.hx                 |   10 +-
 savevm.c                        |   31 +-
 scripts/update-linux-headers.sh |    2 +-
 sysemu.h                        |    4 +-
 umem.c                          |  379 ++++++++
 umem.h                          |  105 +++
 vl.c                            |   20 +-
 30 files changed, 3086 insertions(+), 181 deletions(-)
 create mode 100644 linux-headers/linux/umem.h
 create mode 100644 migration-postcopy-stub.c
 create mode 100644 migration-postcopy.c
 create mode 100644 umem.c
 create mode 100644 umem.h