@node Implementation notes
@appendix Implementation notes

@menu
* Managed start up options::
@end menu

@node Managed start up options
@section Managed start up options

In system mode emulation, it's possible to create a VM in a paused state using
the -S command line option. In this state the machine is completely initialized
according to command line options and ready to execute VM code but VCPU threads
are not executing any code. The VM state in this paused state depends on the way
QEMU was started. It could be in:
@table @asis
@item initial state (after reset/power on state)
@item with direct kernel loading, the initial state could be amended to execute
code loaded by QEMU in the VM's RAM and with incoming migration
@item with incoming migration, initial state will by amended with the migrated
machine state after migration completes.
@end table

This paused state is typically used by users to query machine state and/or
additionally configure the machine (by hotplugging devices) in runtime before
allowing VM code to run.

However, at the -S pause point, it's impossible to configure options that affect
initial VM creation (like: -smp/-m/-numa ...) or cold plug devices. The
experimental --preconfig command line option  allows pausing QEMU
before the initial VM creation, in a ``preconfig'' state, where additional
queries and configuration can be performed via QMP before moving on to
the resulting configuration startup. In the preconfig state, QEMU only allows
a limited set of commands over the QMP monitor, where the commands do not
depend on an initialized machine, including but not limited to:
@table @asis
@item qmp_capabilities
@item query-qmp-schema
@item query-commands
@item query-status
@item x-exit-preconfig
@end table