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diff --git a/qemu-tech.texi b/qemu-tech.texi index 52a56ae25e..dcecba83cb 100644 --- a/qemu-tech.texi +++ b/qemu-tech.texi @@ -5,6 +5,7 @@ * CPU emulation:: * Translator Internals:: * QEMU compared to other emulators:: +* Managed start up options:: * Bibliography:: @end menu @@ -314,6 +315,45 @@ VirtualBox [9], Xen [10] and KVM [11] are based on QEMU. QEMU-SystemC [12] uses QEMU to simulate a system where some hardware devices are developed in SystemC. +@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. That's +when the --preconfig command line option should be used. It allows pausing QEMU +before the initial VM creation, in a new 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 exit-preconfig +@end table +The full list of commands is in QMP schema which could be queried with +query-qmp-schema, where commands supported at preconfig state have option +'allow-preconfig' set to true. + @node Bibliography @section Bibliography |