cli: add --preconfig option

This option allows pausing QEMU in the new RUN_STATE_PRECONFIG state,
allowing the configuration of QEMU from QMP before the machine jumps
into board initialization code of machine_run_board_init()

The intent is to allow management to query machine state and additionally
configure it using previous query results within one QEMU instance
(i.e. eliminate the need to start QEMU twice, 1st to query board specific
parameters and 2nd for actual VM start using query results for
additional parameters).

The new option complements -S option and could be used with or without
it. The difference is that -S pauses QEMU when the machine is completely
initialized with all devices wired up and ready to execute guest code
(QEMU needs only to unpause VCPUs to let guest execute its code),
while the "preconfig" option pauses QEMU early before board specific init
callback (machine_run_board_init) is executed and allows the configuration
of machine parameters which will be used by board init code.

When early introspection/configuration is done, command 'exit-preconfig'
should be used to exit RUN_STATE_PRECONFIG and transition to the next
requested state (i.e. if -S is used then QEMU will pause the second
time when board/device initialization is completed or start guest
execution if -S isn't provided on CLI)

PS:
Initially 'preconfig' is planned to be used for configuring numa
topology depending on board specified possible cpus layout.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1526059483-42847-1-git-send-email-imammedo@redhat.com>
[ehabkost: Changed "since 2.13" to "since 3.0"]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

1 files changed, 40 insertions, 0 deletions

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