doc: update software design guidelines

This patch adds the guidelines for 'Module Level
Configuration Design'.

Signed-off-by: Shiqing Gao <shiqing.gao@intel.com>

Author: shiqingg

doc/developer-guides/images/boot_information_parsing_module.png

@@ -450,6 +450,236 @@ boot information.
 Yes. Because 'vdev->ops' and 'vdev->ops->init' can not be guaranteed to be
 not NULL. If the VM called ``partition_mode_vpci_deinit`` twice, it may be NULL.
 
+
+Module Level Configuration Design Guidelines
+********************************************
+
+Design Goals
+============
+
+There are two goals for module level configuration design, as shown below:
+
+a) In order to make the hypervisor more flexible, one source code and binary
+   is preferred for different platforms with different configurations;
+
+b) If one module is not used by a specific project, the module source code is
+   treated as dead code. The effort to configure it in/out shall be minimized.
+
+
+Hypervisor Operation Modes
+==========================
+
+The hypervisor operation modes are shown in
+:numref:`hypervisor_operation_modes` below.
+
+.. table:: Hypervisor Operation Modes
+   :align: center
+   :widths: 10 10 50
+   :name: hypervisor_operation_modes
+
+   +-------------+-----------+------------------------------------------------------------------------------+
+   | Operation   | Sub-modes | Description                                                                  |
+   | Modes       |           |                                                                              |
+   +=============+===========+==============================================================================+
+   | INIT mode   | DETECT    | The hypervisor detects firmware, detects hardware resource, and reads        |
+   |             | mode      | configuration data.                                                          |
+   |             +-----------+------------------------------------------------------------------------------+
+   |             | STARTUP   | The hypervisor initializes hardware resources, creates virtual resources like|
+   |             | mode      | VCPU and VM, and executes VMLAUNCH instruction(the very first VM entry).     |
+   +-------------+-----------+------------------------------------------------------------------------------+
+   | OPERATIONAL | N/A       | After the first VM entry, the hypervisor runs in VMX root mode and guest OS  |
+   | mode        |           | runs in VMX non-root mode.                                                   |
+   +-------------+-----------+------------------------------------------------------------------------------+
+   | TERMINATION | N/A       | If any fatal error is detected, the hypervisor will enter TERMINATION mode.  |
+   | mode        |           | In this mode, a default fatal error handler will be invoked to handle the    |
+   |             |           | fatal error.                                                                 |
+   +-------------+-----------+------------------------------------------------------------------------------+
+
+
+Configurable Module Properties
+==============================
+
+The properties of configurable modules are shown below:
+
+- The functionality of the module depends on platform configurations;
+- Corresponding platform configurations can be detected in DETECT mode;
+- The module APIs shall be configured in DETECT mode;
+- The module APIs shall be used in modes other than DETECT mode.
+
+Platform configurations include:
+
+- Features depending on hardware or firmware
+- Configuration data provided by firmware
+- Configuration data provided by BSP
+
+
+Design Rules
+============
+
+The module level configuration design rules are shown below:
+
+1. The platform configurations shall be detectable by hypervisor in DETECT mode;
+
+2. Configurable module APIs shall be abstracted as operations which are
+   implemented through a set of function pointers in the operations data
+   structure;
+
+3. Every function pointer in the operations data structure shall be instantiated
+   as one module API in DETECT mode and the API is allowed to be implemented as
+   empty function for some specific configurations;
+
+4. The operations data structure shall be read-only in STARTUP mode, OPERATIONAL
+   mode, and TERMINATION mode;
+
+5. The configurable module shall only be accessed via APIs in the operations
+   data structure in STARTUP mode or OPERATIONAL mode;
+
+6. In order to guarantee that the function pointer in the operations data
+   structure is dereferenced after it has been instantiated, the pre-condition
+   shall be added for the function which deferences the function pointer,
+   instead of checking the pointer for NULL.
+
+.. note:: The third rule shall be double checked during code review.
+
+Use Cases
+=========
+
+The following table shows some use cases of module level configuration design:
+
+.. list-table:: Module Level Configuration Design Use Cases
+   :widths: 10 25 20
+   :header-rows: 1
+
+   * - **Platform Configuration**
+     - **Configurable Module**
+     - **Prerequisite**
+
+   * - Features depending on hardware or firmware
+     - This module is used to virtualize part of LAPIC functionalities.
+       It can be done via APICv or software emulation depending on CPU
+       capabilities.
+       For example, KBL NUC doesn't support virtual-interrupt delivery, while
+       other platforms support it.
+     - If a function pointer is used, the prerequisite is
+       "hv_operation_mode == OPERATIONAL".
+
+   * - Configuration data provided by firmware
+     - This module is used to interact with firmware (UEFI or SBL), and the
+       configuration data is provided by firmware.
+       For example, UP2 uses SBL and KBL NUC uses UEFI.
+     - If a function pointer is used, the prerequisite is
+       "hv_operation_mode != DETECT".
+
+   * - Configuration data provided by BSP
+     - This module is used to virtualize LAPIC, and the configuration data is
+       provided by BSP.
+       For example, some VMs use LAPIC pass-through and the other VMs use
+       vLAPIC.
+     - If a function pointer is used, the prerequisite is
+       "hv_operation_mode == OPERATIONAL".
+
+.. note:: Prerequisite is used to guarantee that the function pointer used for
+   configuration is dereferenced after it has been instantiated.
+
+
+Examples
+========
+
+Take the module for parsing boot information as an example to illustrate the
+idea of module level configuration design.
+
+.. figure:: images/boot_information_parsing_module.png
+   :align: center
+   :scale: 70 %
+   :name: boot_information_parsing_module
+
+   Boot Information Parsing Module
+
+
+As shown in the source code below, 'struct firmware_operations' is an operations
+data structure that contains a set of function pointers.
+Different firmware may have different implementations:
+
+- 'firmware_uefi_ops' is for UEFI platform;
+- 'firmware_sbl_ops' is for SBL platform.
+
+
+.. code-block:: c
+
+  struct firmware_operations {
+          void (*init)(void);
+          uint64_t (*get_ap_trampoline)(void);
+          void *(*get_rsdp)(void);
+          void (*init_irq)(void);
+          int32_t (*init_vm_boot_info)(struct acrn_vm *vm);
+  };
+
+  static struct firmware_operations firmware_uefi_ops = {
+          .init = uefi_init,
+          .get_ap_trampoline = uefi_get_ap_trampoline,
+          .get_rsdp = uefi_get_rsdp,
+          .init_irq = uefi_init_irq,
+          .init_vm_boot_info = uefi_init_vm_boot_info,
+  };
+
+  static struct firmware_operations firmware_sbl_ops = {
+          .init = sbl_init,
+          .get_ap_trampoline = sbl_get_ap_trampoline,
+          .get_rsdp = sbl_get_rsdp,
+          .init_irq = sbl_init_irq,
+          .init_vm_boot_info = sbl_init_vm_boot_info,
+  };
+
+
+'firmware_ops' is the operations set that is dereferenced and takes effect.
+
+'init_firmware_operations' is called when the hypervisor is in DETECT mode and
+'firmware_ops' is instantiated here to either 'firmware_uefi_ops' or
+'firmware_sbl_ops' depending on the platform.
+
+.. note:: All the other exported interfaces using 'firmware_ops' shall be called
+   after the instantiation.
+
+
+.. code-block:: c
+
+  static struct firmware_operations *firmware_ops;
+
+  struct firmware_operations* uefi_get_firmware_operations(void)
+  {
+          return &firmware_uefi_ops;
+  }
+
+  struct firmware_operations* sbl_get_firmware_operations(void)
+  {
+          return &firmware_sbl_ops;
+  }
+
+  void init_firmware_operations(void)
+  {
+          if (is_firmware_sbl()) {
+                  firmware_ops = sbl_get_firmware_operations();
+          } else {
+                  firmware_ops = uefi_get_firmware_operations();
+          }
+  }
+
+
+For example, when the hypervisor needs to initialize the VM boot information,
+it calls 'firmware_init_vm_boot_info' and 'firmware_ops->init_vm_boot_info' is
+dereferenced here with correct API being called.
+
+.. code-block:: c
+
+  /**
+   * @pre firmware_ops->init_vm_boot_info != NULL
+   */
+  int32_t firmware_init_vm_boot_info(struct acrn_vm *vm)
+  {
+          return firmware_ops->init_vm_boot_info(vm);
+  }
+
+
 References
 **********