acrn-hypervisor/doc/developer-guides/modularity.rst

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.. _modularity:
ACRN Hypervisor: Modular Design
###############################
Overview
********
ACRN highly emphasizes modular design, i.e. the separation of functionality info
modules that define a concise set of interfaces. The goals of modular design
include:
* **Understandability** A modular design is easier to understand due to
encapsulation.
* **Testability** Modules can be integrated and tested in the reverse order of
dependencies among them. White-box integration tests help improve the coverage
of tests and identify corner cases that are hard to trigger when testing the
hypervisor as a whole.
* **Configurability** Modular design makes it easy to configure certain
functionalities in or out. This is crucial in safety-critical scenarios
because absence of irrelevant code is required in both MISRA-C and functional
safety standards.
* **Meet functional safety requirements** Functional safety standards explicitly
require a hierarchical structure of modules in software architectural
design. This applies to any safety integrity level defined in
[IEC_61508-3]_ and [ISO_26262-6]_.
Principles
**********
* Each source file shall belong to one module only. One module may consist of
one or multiple source files, though. A source file can be a C source, a C
header or an assembly file.
* Each module shall have well-defined interfaces, including the exported
functions and global variables. Functions and variables that are not
interfaces shall be static and used inside the module only.
* Dependencies among the modules should be acyclic. Any cyclic dependency must
be deviated explicitly.
* The complexity of a module shall be limited.
Minimizing Cyclic Dependencies
==============================
Cyclic dependencies can be mostly avoided by carefully defining the boundary of
modules. The following methods can be used when certain cyclic dependency cannot
be resolved by design.
* **Use callbacks** Callback registration and invocation help reverse dependencies
between two modules and break cyclic dependencies. However callbacks shall be
used with care due to its dynamic behavior. Send proposals or patches to the
`acrn-dev mailing list <https://lists.projectacrn.org/g/acrn-dev>`_ for
discussing if specific callbacks are appropriate.
* **Making the cyclic dependency an exception** A specific cyclic dependency can
be regarded as an exception if it is well justified and a work around is
available to break the cyclic dependency for integration testing.
Measuring Complexity
====================
ACRN uses the number of functions and the cyclomatic complexity [CC]_ of each
function to measure the complexity of a module. Concrete criteria on complexity
will be determined while enhancing the modularity of the hypervisor. The current
recommendation is to limit the cyclomatic complexity of a function under 20.
Architecture
************
The following figure shows the high-level components of ACRN hypervisor.
.. figure:: images/modularity-architecture.png
:align: center
:name: modularity-architecture
Layered Architecture of ACRN Hypervisor
The components are listed as follows.
* **Boot** This component carries out the most basic hardware initialization to
enable the execution of C code.
* **Library** This component consists of subroutines that require no explicit
initialization. Examples include standard memory and string manipulation
functions like strncpy, atomic operations and bitmap operations. This
component is independent from and widely used in the other components.
* **Hardware Management and Utilities** This component abstract hardware
resources and provide services like timers and physical interrupt handler
registration to the upper layers.
* **Virtual CPU** This component implements CPU, memory and interrupt
virtualization. The vCPU loop module in this component handles VM exit events
by calling the proper handler in the other components. Hypercalls are
implemented as a special type of VM exit event. This component is also able to
inject upcall interrupts to the Service VM.
* **Device Emulation** This component implements devices that are emulated in
the hypervisor itself, such as the virtual programmable interrupt controllers
including vPIC, vLAPIC and vIOAPIC.
* **Passthru Management** This component manages devices that are passed-through
to specific VMs.
* **Extended Device Emulation** This component implements an I/O request
mechanism that allow the hypervisor to forward I/O accesses from a User
VM to the Service VM.
for emulation.
* **VM Management** This component manages the creation, deletion and other
lifecycle operations of VMs.
* **Hypervisor Initialization** This component invokes the initialization
subroutines in the other components to bring up the hypervisor and start up
Service VM in sharing mode or all the VMs in partitioning mode.
ACRN hypervisor adopts a layered design where higher layers can invoke the
interfaces of lower layers but not vice versa. The only exception is the
invocation of initialization routine in the **Boot** component, illustrated as
the arrow from bottom to top on the left side of figure
:numref:`modularity-architecture`. This exception is made due to the following
reasons.
* **Boot** enables the execution of C code and thus has to be the lowest layer
in the architecture.
* **Hypervisor Initialization** contains the hypervisor initialization function
that calls the initialization functions of each layer. Thus this component is
the highest layer to minimize reverse dependencies.
* **Boot** shall invoke the hypervisor initialization routine after bringing up
the hardware. This inevitably causes a reverse dependency from **Boot** to
**Hypervisor Initialization**.
To enable integration testing of a layer in the middle (e.g. **Virtual CPU**),
**Boot** will invoke a customized function that only invokes the initialization
functions of that layer as well as the layers below.
References
**********
.. [IEC_61508-3] IEC 61508-3:2010, Functional safety of electrical/electronic/programmable electronic safety-related systems - Part 3: Software requirements
.. [ISO_26262-6] ISO 26262-6:2011, Road vehicles - Functional safety - Part 6: Product development at the software level
.. [CC] Cyclomatic complexity - Wikipedia, https://en.wikipedia.org/wiki/Cyclomatic_complexity