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

[Peter Xu]

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?

>   * 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
> 

-- 
Peter Xu