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+/* $Id: Docs-Intro.cpp $ */
+/** @file
+ * This file contains the introduction to Main for developers.
+ */
+
+/*
+ * Copyright (C) 2011-2022 Oracle and/or its affiliates.
+ *
+ * This file is part of VirtualBox base platform packages, as
+ * available from https://www.virtualbox.org.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, in version 3 of the
+ * License.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <https://www.gnu.org/licenses>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+/** @page pg_main       Main API
+ *
+ * First of all, check out the "Technical background" chapter in the manual, pay
+ * attention to the "VirtualBox executables and components" chapter.  It lists
+ * three processes, (1) VBoxSVC, (2) VirtualBox in manager mode and (3)
+ * VirtualBox in VM mode.  This will be referred to as (1) server, (2) client
+ * and (3) VM process, respectively.
+ *
+ *
+ * @section sec_main_walk_thru_suspend  IConsole::Pause Walkthru
+ *
+ * The instigator can be a client (VirtualBox in manager mode, VBoxManage
+ * controlvm, web services, ++) or the VM process it self (i.e. you select
+ * pause via the menu or the host key short cut).
+ *
+ * We will not cover the case where the guest triggers a suspend.
+ *
+ * Approximate sequence of events:
+ *  - Client calls IConsole::Pause.
+ *  - The COM/XPCOM routes this to the VM process, invoking Console::Pause() in
+ *    ConsoleImpl.cpp. (The IConsole::Pause method in the client process is a
+ *    COM/XPCOM stub method which does marshalling+IPC.)
+ *  - Console::Pause validates the Console object state, the VM state and the VM
+ *    handle.
+ *  - Console::Pause calls VMR3Suspend to do the actual suspending.
+ *  - VMR3Suspend() in VMM/VMMR3/VM.cpp calls VMMR3EmtRendezvous() to change the
+ *    VM state synchronously on all EMTs (threads performing as virtual CPUs).
+ *  - VMMR3EmtRendezvous() will first detect that the caller isn't an EMT and
+ *    use VMR3ReqCallWait() to forward the call to an EMT.
+ *  - When VMMR3EmtRendezvous() is called again on an EMT, it will signal the
+ *    other EMTs by raising a force action flag (VM_FF_EMT_RENDEZVOUS) and then
+ *    poke them via VMR3NotifyGlobalFFU(). Then wait for them all to arrive.
+ *  - The other EMTs will call VMMR3EmtRendezvousFF as soon as they can.
+ *  - When all EMTs are there, the calling back of vmR3Suspend() on each CPU in
+ *    decending order will start.
+ *  - When the CPU with the higest ID calls vmR3Suspend() the VM state is
+ *    changed to VMSTATE_SUSPENDING or VMSTATE_SUSPENDING_EXT_LS.
+ *  - When the CPU with ID 0 calls vmR3Suspend() the virtual device emulations
+ *    and drivers get notified via PDMR3Suspend().
+ *  - PDMR3Suspend() in VMM/VMMR3/PDM.cpp will iterate thru all device
+ *    emulations and notify them that the VM is suspending by calling their
+ *    PDMDEVREG::pfnSuspend / PDMUSBREG::pfnSuspend entry point (can be NULL).
+ *    For each device it will iterate the chains of drivers and call their
+ *    PDMDRVREG::pfnSuspend entry point as well.
+ *  - Should a worker thread in a PDM device or PDM driver be busy and need some
+ *    extra time to finish up / notice the pending suspend, the device or driver
+ *    will ask for more time via PDMDevHlpSetAsyncNotification(),
+ *    PDMDrvHlpSetAsyncNotification() or PDMUsbHlpSetAsyncNotification().
+ *    PDMR3Suspend will then poll these devices and drivers frequently until all
+ *    are done.
+ *  - PDMR3Suspend() will return to vmR3Suspend() once all PDM devices and PDM
+ *    drivers has responded to the pfnSuspend callback.
+ *  - The virtual CPU with ID 0 returns from vmR3Suspend() to the rendezvous
+ *    code and the EMTs are released.
+ *  - The inner VMMR3EmtRendezvous() call returns and this in turn triggers the
+ *    VMR3ReqCallWait() call to return (with the status code of the inner call).
+ *  - The outer VMMR3EmtRendezvous() returns to VMR3Suspend().
+ *  - VMR3Suspend() returns to Console::Pause().
+ *  - Console::Pause() checks the result and flags provides error details on
+ *    failure.
+ *  - Console::Pause() returns to the COM/XPCOM marshalling/IPC stuff.
+ *  - Switch back to client process.
+ *  - The IConsole::Pause() call returns. The end.
+ *
+ * Summary of above: Client process calls into the VM process, VM process does a
+ * bunch of inter thread calls with all the EMT, EMT0 suspends the PDM devices
+ * and drivers.
+ *
+ * The EMTs will return to the outer execution loop, vmR3EmulationThreadWithId()
+ * in VMM/VMMR3/VMEmt.cpp, where they will mostly do sleep.  They will not
+ * execute any guest code until VMR3Resume() is called.
+ *
+ */
+