Steve Sistare <[email protected]> writes: > Update developer documentation for cpr-exec mode. > > Signed-off-by: Steve Sistare <[email protected]>
Reviewed-by: Fabiano Rosas <[email protected]> Just a typo below. > --- > docs/devel/migration/CPR.rst | 103 > ++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 102 insertions(+), 1 deletion(-) > > diff --git a/docs/devel/migration/CPR.rst b/docs/devel/migration/CPR.rst > index 0a0fd4f..abc9a90 100644 > --- a/docs/devel/migration/CPR.rst > +++ b/docs/devel/migration/CPR.rst > @@ -5,7 +5,7 @@ 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, > -the cpr-reboot and cpr-transfer modes are available. > +the cpr-reboot, cpr-transfer, and cpr-exec modes are available. > > Because QEMU is restarted on the same host, with access to the same > local devices, CPR is allowed in certain cases where normal migration > @@ -324,3 +324,104 @@ descriptors from old to new QEMU. In the future, > descriptors for > vhost, and char devices could be transferred, > preserving those devices and their kernel state without interruption, > even if they do not explicitly support live migration. > + > +cpr-exec mode > +------------- > + > +In this mode, QEMU stops the VM, writes VM state to the migration > +URI, and directly exec's a new version of QEMU on the same host, > +replacing the original process while retaining its PID. Guest RAM is > +preserved in place, albeit with new virtual addresses. The user > +completes the migration by specifying the ``-incoming`` option, and > +by issuing the ``migrate-incoming`` command if necessary; see details > +below. > + > +This mode supports VFIO/IOMMUFD devices by preserving device descriptors > +and hence kernel state across the exec, even for devices that do not > +support live migration. > + > +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. > + > +Usage > +^^^^^ > + > +Arguments for the new QEMU process are taken from the > +@cpr-exec-args parameter. The first argument should be the > +path of a new QEMU binary, or a prefix command that exec's the > +new QEMU binary, and the arguments should include the ''-incoming'' > +option. > + > +Memory backend objects must have the ``share=on`` attribute. > +The VM must be started with the ``-machine aux-ram-share=on`` option. > + > +Outgoing: > + * Set the migration mode parameter to ``cpr-exec``. > + * Set the ``cpr-exec-args`` parameter. > + * Issue the ``migrate`` command. It is recommended the the URI be s/the the/that the/ > + 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. > + > +Incoming: > + * You do not need to explicitly start new QEMU. It is started as > + a side effect of the migrate command above. > + * If the VM was running when the outgoing ``migrate`` command was > + issued, then QEMU automatically resumes VM execution. > + > +Example 1: incoming URI > +^^^^^^^^^^^^^^^^^^^^^^^ > + > +In these examples, we simply restart the same version of QEMU, but in > +a real scenario one would set a new QEMU binary path in cpr-exec-args. > + > +:: > + > + # qemu-kvm -monitor stdio > + -object memory-backend-memfd,id=ram0,size=4G > + -machine memory-backend=ram0 > + -machine aux-ram-share=on > + ... > + > + QEMU 10.2.50 monitor - type 'help' for more information > + (qemu) info status > + VM status: running > + (qemu) migrate_set_parameter mode cpr-exec > + (qemu) migrate_set_parameter cpr-exec-args qemu-kvm ... -incoming > file:vm.state > + (qemu) migrate -d file:vm.state > + (qemu) QEMU 10.2.50 monitor - type 'help' for more information > + (qemu) info status > + VM status: running > + > +Example 2: incoming defer > +^^^^^^^^^^^^^^^^^^^^^^^^^ > +:: > + > + # qemu-kvm -monitor stdio > + -object memory-backend-memfd,id=ram0,size=4G > + -machine memory-backend=ram0 > + -machine aux-ram-share=on > + ... > + > + QEMU 10.2.50 monitor - type 'help' for more information > + (qemu) info status > + VM status: running > + (qemu) migrate_set_parameter mode cpr-exec > + (qemu) migrate_set_parameter cpr-exec-args qemu-kvm ... -incoming defer > + (qemu) migrate -d file:vm.state > + (qemu) QEMU 10.2.50 monitor - type 'help' for more information > + (qemu) info status > + status: paused (inmigrate) > + (qemu) migrate_incoming file:vm.state > + (qemu) info status > + VM status: running > + > +Caveats > +^^^^^^^ > + > +cpr-exec mode may not be used with postcopy, background-snapshot, > +or COLO. > + > +cpr-exec mode requires permission to use the exec system call, which > +is denied by certain sandbox options, such as spawn.
