Re: [RFC V1 0/6] Live update: cpr-transfer

[Steven Sistare]

On 7/18/2024 11:36 AM, Peter Xu wrote:
> On Sun, Jun 30, 2024 at 12:44:02PM -0700, Steve Sistare wrote:
> > What?
> >
> > This patch series adds the live migration cpr-transfer mode, which
> > allows the user to transfer a guest to a new QEMU instance on the same
> > host.  It is identical to cpr-exec in most respects, except as described
> > below.
>
> I definitely prefer this one more than the exec solution, thanks for trying
> this out.  It's a matter of whether we'll need both, my answer would be
> no..
>
> > The new user-visible interfaces are:
> >    * cpr-transfer (MigMode migration parameter)
> >    * cpr-uri (migration parameter)
>
> I wonder whether this parameter can be avoided already, maybe we can let
> cpr-transfer depend on unix socket in -incoming, then integrate fd sharing
> in the same channel?

You saw the answer in another thread, but I repeat it here for others benefit:

  "CPR state cannot be sent over the normal migration channel, because devices
   and backends are created prior to reading the channel, so this mode sends
   CPR state over a second migration channel that is not visible to the user.
   New QEMU reads the second channel prior to creating devices or backends."

- Steve

> >    * cpr-uri (command-line argument)
> >
> > In this mode, the user starts new QEMU on the same host as old QEMU, with
> > the same arguments as old QEMU, plus the -incoming and the -cpr-uri options.
> > The user issues the migrate command to old QEMU, which stops the VM, saves
> > state to the migration channels, and enters the postmigrate state.  Execution
> > resumes in new QEMU.
> >
> > This mode requires a second migration channel, specified by the cpr-uri
> > migration property on the outgoing side, and by the cpr-uri QEMU command-line
> > option on the incoming side.  The channel must be a type, such as unix socket,
> > that supports SCM_RIGHTS.
> >
> > This series depends on the series "Live update: cpr-exec mode".
> >
> > Why?
> >
> > cpr-transfer offers the same benefits as cpr-exec mode, but with a model
> > for launching new QEMU that may be more natural for some management packages.
> >
> > How?
> >
> > The file descriptors are kept open by sending them to new QEMU via the
> > cpr-uri, which must support SCM_RIGHTS.
> >
> > Example:
> >
> > In this example, we simply restart the same version of QEMU, but in
> > a real scenario one would use a new QEMU binary path in terminal 2.
> >
> >    Terminal 1: start old QEMU
> >    # qemu-kvm -monitor stdio -object
> >    memory-backend-file,id=ram0,size=4G,mem-path=/dev/shm/ram0,share=on
> >    -m 4G -machine anon-alloc=memfd ...
> >
> >    Terminal 2: start new QEMU
> >    # qemu-kvm ... -incoming unix:vm.sock -cpr-uri unix:cpr.sock
> >
> >    Terminal 1:
> >    QEMU 9.1.50 monitor - type 'help' for more information
> >    (qemu) info status
> >    VM status: running
> >    (qemu) migrate_set_parameter mode cpr-transfer
> >    (qemu) migrate_set_parameter cpr-uri unix:cpr.sock
> >    (qemu) migrate -d unix:vm.sock
> >    (qemu) info status
> >    VM status: paused (postmigrate)
> >
> >    Terminal 2:
> >    QEMU 9.1.50 monitor - type 'help' for more information
> >    (qemu) info status
> >    VM status: running
> >
> > Steve Sistare (6):
> >    migration: SCM_RIGHTS for QEMUFile
> >    migration: VMSTATE_FD
> >    migration: cpr-transfer save and load
> >    migration: cpr-uri parameter
> >    migration: cpr-uri option
> >    migration: cpr-transfer mode
> >
> >   include/migration/cpr.h        |  4 ++
> >   include/migration/vmstate.h    |  9 +++++
> >   migration/cpr-transfer.c       | 81 +++++++++++++++++++++++++++++++++++++++++
> >   migration/cpr.c                | 16 +++++++-
> >   migration/meson.build          |  1 +
> >   migration/migration-hmp-cmds.c | 10 +++++
> >   migration/migration.c          | 37 +++++++++++++++++++
> >   migration/options.c            | 29 +++++++++++++++
> >   migration/options.h            |  1 +
> >   migration/qemu-file.c          | 83 ++++++++++++++++++++++++++++++++++++++++--
> >   migration/qemu-file.h          |  2 +
> >   migration/ram.c                |  1 +
> >   migration/trace-events         |  2 +
> >   migration/vmstate-types.c      | 33 +++++++++++++++++
> >   qapi/migration.json            | 38 ++++++++++++++++++-
> >   qemu-options.hx                |  8 ++++
> >   stubs/vmstate.c                |  7 ++++
> >   system/vl.c                    |  3 ++
> >   18 files changed, 359 insertions(+), 6 deletions(-)
> >   create mode 100644 migration/cpr-transfer.c
> >
> > --
> > 1.8.3.1