Re: [PATCH] migration: cpr-reboot documentation

["Cédric Le Goater" <clg@redhat.com>]

On 3/13/24 14:55, Steve Sistare wrote:
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>

This would be good to have for 9.0. Anyhow,

Reviewed-by: Cédric Le Goater <clg@redhat.com>

Thanks,

C.


> ---
>   docs/devel/migration/CPR.rst      | 147 ++++++++++++++++++++++++++++++++++++++
>   docs/devel/migration/features.rst |   1 +
>   2 files changed, 148 insertions(+)
>   create mode 100644 docs/devel/migration/CPR.rst
> 
> diff --git a/docs/devel/migration/CPR.rst b/docs/devel/migration/CPR.rst
> new file mode 100644
> index 0000000..63c3647
> --- /dev/null
> +++ b/docs/devel/migration/CPR.rst
> @@ -0,0 +1,147 @@
> +CheckPoint and Restart (CPR)
> +============================
> +
> +CPR is the umbrella name for a set of migration modes in which the
> +VM is migrated to a new QEMU instance on the same host.  It is
> +intended for use when the goal is to update host software components
> +that run the VM, such as QEMU or even the host kernel.  At this time,
> +cpr-reboot is the only available mode.
> +
> +Because QEMU is restarted on the same host, with access to the same
> +local devices, CPR is allowed in certain cases where normal migration
> +would be blocked.  However, the user must not modify the contents of
> +guest block devices between quitting old QEMU and starting new QEMU.
> +
> +CPR unconditionally stops VM execution before memory is saved, and
> +thus does not depend on any form of dirty page tracking.
> +
> +cpr-reboot mode
> +---------------
> +
> +In this mode, QEMU stops the VM, and writes VM state to the migration
> +URI, which will typically be a file.  After quitting QEMU, the user
> +resumes by running QEMU with the ``-incoming`` option.  Because the
> +old and new QEMU instances are not active concurrently, the URI cannot
> +be a type that streams data from one instance to the other.
> +
> +Guest RAM can be saved in place if backed by shared memory, or can be
> +copied to a file.  The former is more efficient and is therefore
> +preferred.
> +
> +After state and memory are saved, the user may update userland host
> +software before restarting QEMU and resuming the VM.  Further, if
> +the RAM is backed by persistent shared memory, such as a DAX device,
> +then the user may reboot to a new host kernel before restarting QEMU.
> +
> +This mode supports VFIO devices provided the user first puts the
> +guest in the suspended runstate, such as by issuing the
> +``guest-suspend-ram`` command to the QEMU guest agent.  The agent
> +must be pre-installed in the guest, and the guest must support
> +suspend to RAM.  Beware that suspension can take a few seconds, so
> +the user should poll to see the suspended state before proceeding
> +with the CPR operation.
> +
> +Usage
> +^^^^^
> +
> +It is recommended that guest RAM be backed with some type of shared
> +memory, such as ``memory-backend-file,share=on``, and that the
> +``x-ignore-shared`` capability be set.  This combination allows memory
> +to be saved in place.  Otherwise, after QEMU stops the VM, all guest
> +RAM is copied to the migration URI.
> +
> +Outgoing:
> +  * Set the migration mode parameter to ``cpr-reboot``.
> +  * Set the ``x-ignore-shared`` capability if desired.
> +  * Issue the ``migrate`` command.  It is recommended the the URI be a
> +    ``file`` type, but one can use other types such as ``exec``,
> +    provided the command captures all the data from the outgoing side,
> +    and provides all the data to the incoming side.
> +  * Quit when QEMU reaches the postmigrate state.
> +
> +Incoming:
> +  * Start QEMU with the ``-incoming defer`` option.
> +  * Set the migration mode parameter to ``cpr-reboot``.
> +  * Set the ``x-ignore-shared`` capability if desired.
> +  * Issue the ``migrate-incoming`` command.
> +  * If the VM was running when the outgoing ``migrate`` command was
> +    issued, then QEMU automatically resumes VM execution.
> +
> +Example 1
> +^^^^^^^^^
> +::
> +
> +  # qemu-kvm -monitor stdio
> +  -object memory-backend-file,id=ram0,size=4G,mem-path=/dev/dax0.0,align=2M,share=on -m 4G
> +  ...
> +
> +  (qemu) info status
> +  VM status: running
> +  (qemu) migrate_set_parameter mode cpr-reboot
> +  (qemu) migrate_set_capability x-ignore-shared on
> +  (qemu) migrate -d file:vm.state
> +  (qemu) info status
> +  VM status: paused (postmigrate)
> +  (qemu) quit
> +
> +  ### optionally update kernel and reboot
> +  # systemctl kexec
> +  kexec_core: Starting new kernel
> +  ...
> +
> +  # qemu-kvm ... -incoming defer
> +  (qemu) info status
> +  VM status: paused (inmigrate)
> +  (qemu) migrate_set_parameter mode cpr-reboot
> +  (qemu) migrate_set_capability x-ignore-shared on
> +  (qemu) migrate_incoming file:vm.state
> +  (qemu) info status
> +  VM status: running
> +
> +Example 2: VFIO
> +^^^^^^^^^^^^^^^
> +::
> +
> +  # qemu-kvm -monitor stdio
> +  -object memory-backend-file,id=ram0,size=4G,mem-path=/dev/dax0.0,align=2M,share=on -m 4G
> +  -device vfio-pci, ...
> +  -chardev socket,id=qga0,path=qga.sock,server=on,wait=off
> +  -device virtserialport,chardev=qga0,name=org.qemu.guest_agent.0
> +  ...
> +
> +  (qemu) info status
> +  VM status: running
> +
> +  # echo '{"execute":"guest-suspend-ram"}' | ncat --send-only -U qga.sock
> +
> +  (qemu) info status
> +  VM status: paused (suspended)
> +  (qemu) migrate_set_parameter mode cpr-reboot
> +  (qemu) migrate_set_capability x-ignore-shared on
> +  (qemu) migrate -d file:vm.state
> +  (qemu) info status
> +  VM status: paused (postmigrate)
> +  (qemu) quit
> +
> +  ### optionally update kernel and reboot
> +  # systemctl kexec
> +  kexec_core: Starting new kernel
> +  ...
> +
> +  # qemu-kvm ... -incoming defer
> +  (qemu) info status
> +  VM status: paused (inmigrate)
> +  (qemu) migrate_set_parameter mode cpr-reboot
> +  (qemu) migrate_set_capability x-ignore-shared on
> +  (qemu) migrate_incoming file:vm.state
> +  (qemu) info status
> +  VM status: paused (suspended)
> +  (qemu) system_wakeup
> +  (qemu) info status
> +  VM status: running
> +
> +Caveats
> +^^^^^^^
> +
> +cpr-reboot mode may not be used with postcopy, background-snapshot,
> +or COLO.
> diff --git a/docs/devel/migration/features.rst b/docs/devel/migration/features.rst
> index 9d1abd2..d5ca7b8 100644
> --- a/docs/devel/migration/features.rst
> +++ b/docs/devel/migration/features.rst
> @@ -11,3 +11,4 @@ Migration has plenty of features to support different use cases.
>      vfio
>      virtio
>      mapped-ram
> +   CPR