mirror of
https://gitlab.com/libvirt/libvirt.git
synced 2024-11-03 11:51:11 +00:00
303 lines
12 KiB
HTML
303 lines
12 KiB
HTML
<html>
|
|
<head>
|
|
<title>Implementing a new API in Libvirt</title>
|
|
</head>
|
|
|
|
<body>
|
|
<h1>Implementing a new API in Libvirt</h1>
|
|
|
|
<ul id="toc"></ul>
|
|
|
|
<p>
|
|
This document walks you through the process of implementing a new
|
|
API in libvirt. It uses as an example the addition of the node device
|
|
create and destroy APIs.
|
|
</p>
|
|
|
|
<p>
|
|
Before you begin coding, it is critical that you propose your
|
|
changes on the libvirt mailing list and get feedback on your ideas to
|
|
make sure what you're proposing fits with the general direction of the
|
|
project. Even before doing a proof of concept implementation, send an
|
|
email giving an overview of the functionality you think should be
|
|
added to libvirt. Someone may already be working on the feature you
|
|
want. Also, recognize that everything you write is likely to undergo
|
|
significant rework as you discuss it with the other developers, so
|
|
don't wait too long before getting feedback.
|
|
</p>
|
|
|
|
<p>
|
|
Adding a new API to libvirt is not difficult, but there are quite a
|
|
few steps. This document assumes that you are familiar with C
|
|
programming and have checked out the libvirt code from the source code
|
|
repository and successfully built the existing tree. Instructions on
|
|
how to check out and build the code can be found at:
|
|
</p>
|
|
|
|
<p>
|
|
<a href="http://libvirt.org/downloads.html">http://libvirt.org/downloads.html</a>
|
|
</p>
|
|
|
|
<p>
|
|
Once you have a working development environment, the steps to create a
|
|
new API are:
|
|
</p>
|
|
<ol>
|
|
<li>define the public API</li>
|
|
<li>define the internal driver API</li>
|
|
<li>implement the public API</li>
|
|
<li>define the wire protocol format</li>
|
|
<li>implement the RPC client</li>
|
|
<li>implement the server side dispatcher</li>
|
|
<li>implement the driver methods</li>
|
|
<li>add virsh support</li>
|
|
</ol>
|
|
|
|
<p>
|
|
It is, of course, possible to implement the pieces in any order, but
|
|
if the development tasks are completed in the order listed, the code
|
|
will compile after each step. Given the number of changes required,
|
|
verification after each step is highly recommended.
|
|
</p>
|
|
|
|
<p>
|
|
Submit new code in the form shown in the example code: one patch
|
|
per step. That's not to say submit patches before you have working
|
|
functionality--get the whole thing working and make sure you're happy
|
|
with it. Then use git or some other version control system that lets
|
|
you rewrite your commit history and break patches into pieces so you
|
|
don't drop a big blob of code on the mailing list at one go. For
|
|
example, I didn't follow my own advice when I originally submitted the
|
|
example code to the libvirt list but rather submitted it in several
|
|
large chunks. I've used git's ability to rewrite my commit history to
|
|
break the code apart into the example patches shown.
|
|
</p>
|
|
|
|
<p>
|
|
Don't mix anything else into the patches you submit. The patches
|
|
should be the minimal changes required to implement the functionality
|
|
you're adding. If you notice a bug in unrelated code (i.e., code you
|
|
don't have to touch to implement your API change) during development,
|
|
create a patch that just addresses that bug and submit it
|
|
separately.
|
|
</p>
|
|
|
|
<p>With that said, let's begin.</p>
|
|
|
|
<h2><a name='publicapi'>Defining the public API</a></h2>
|
|
|
|
<p>The first task is to define the public API and add it to:</p>
|
|
|
|
<p><code>include/libvirt/libvirt.h.in</code></p>
|
|
|
|
<p>
|
|
This task is in many ways the most important to get right, since once
|
|
the API has been committed to the repository, it's libvirt's policy
|
|
never to change it. Mistakes in the implementation are bugs that you
|
|
can fix. Make a mistake in the API definition and you're stuck with
|
|
it, so think carefully about the interface and don't be afraid to
|
|
rework it as you go through the process of implementing it.
|
|
</p>
|
|
|
|
<p>Once you have defined the API, you have to add the symbol names to:</p>
|
|
|
|
<p><code>src/libvirt_public.syms</code></p>
|
|
|
|
<p class="example">See <a href="api_extension/0001-Step-1-of-8-Define-the-public-API.patch">0001-Step-1-of-8-Define-the-public-API.patch</a> for example code.</p>
|
|
|
|
|
|
<h2><a name='internalapi'>Defining the internal API</a></h2>
|
|
|
|
<p>
|
|
Each public API call is associated with a driver, such as a host
|
|
virtualization driver, a network virtualization driver, a storage
|
|
virtualization driver, a state driver, or a device monitor. Adding
|
|
the internal API is ordinarily a matter of adding a new member to the
|
|
struct representing one of these drivers.
|
|
</p>
|
|
|
|
<p>
|
|
Of course, it's possible that the new API will involve the creation of
|
|
an entire new driver type, in which case the changes will include the
|
|
creation of a new struct type to represent the new driver type.
|
|
</p>
|
|
|
|
<p>The driver structs are defined in:</p>
|
|
|
|
<p><code>src/driver.h</code></p>
|
|
|
|
<p>
|
|
To define the internal API, first typedef the driver function
|
|
prototype and then add a new field for it to the relevant driver
|
|
struct.
|
|
</p>
|
|
|
|
<p class="example">See <a href="api_extension/0002-Step-2-of-8-Define-the-internal-driver-API.patch">0002-Step-2-of-8-Define-the-internal-driver-API.patch</a></p>
|
|
|
|
<h2><a name='implpublic'>Implementing the public API</a></h2>
|
|
|
|
<p>
|
|
Implementing the public API is largely a formality in which we wire up
|
|
public API to the internal driver API. The public API implementation
|
|
takes care of some basic validity checks before passing control to the
|
|
driver implementation. In RFC 2119 vocabulary, this function:
|
|
</p>
|
|
|
|
<ol class="ordinarylist">
|
|
<li>SHOULD log a message with VIR_DEBUG() indicating that it is
|
|
being called and its parameters;</li>
|
|
<li>MUST call virResetLastError();</li>
|
|
<li>SHOULD confirm that the connection is valid with
|
|
VIR_IS_CONNECT(conn);</li>
|
|
<li><strong>SECURITY: If the API requires a connection with write
|
|
privileges, MUST confirm that the connection flags do not
|
|
indicate that the connection is read-only;</strong></li>
|
|
<li>SHOULD do basic validation of the parameters that are being
|
|
passed in;</li>
|
|
<li>MUST confirm that the driver for this connection exists and that
|
|
it implements this function;</li>
|
|
<li>MUST call the internal API;</li>
|
|
<li>SHOULD log a message with VIR_DEBUG() indicating that it is
|
|
returning, its return value, and status.</li>
|
|
<li>MUST return status to the caller.</li>
|
|
</ol>
|
|
|
|
<p>The public API calls are implemented in:</p>
|
|
|
|
<p><code>src/libvirt.c</code></p>
|
|
|
|
<p class="example">See <a href="api_extension/0003-Step-3-of-8-Implement-the-public-API.patch">0003-Step-3-of-8-Implement-the-public-API.patch</a></p>
|
|
|
|
|
|
<h2><a name='wireproto'>Defining the wire protocol format</a></h2>
|
|
|
|
<p>
|
|
Defining the wire protocol is essentially a straightforward exercise
|
|
which is probably most easily understood by referring to the existing
|
|
remote protocol wire format definitions and the example patch. It
|
|
involves making two additions to:
|
|
</p>
|
|
|
|
<p><code>src/remote/remote_protocol.x</code></p>
|
|
|
|
<p>
|
|
First, create two new structs for each new function that you're adding
|
|
to the API. One struct describes the parameters to be passed to the
|
|
remote function, and a second struct describes the value returned by
|
|
the remote function. The one exception to this rule is that functions
|
|
that return only integer status do not require a struct for returned
|
|
data.
|
|
</p>
|
|
|
|
<p>
|
|
Second, add values to the remote_procedure enum for each new function
|
|
added to the API.
|
|
</p>
|
|
|
|
<p class="example">See <a href="api_extension/0004-Step-4-of-8-Define-the-wire-protocol-format.patch">0004-Step-4-of-8-Define-the-wire-protocol-format.patch</a></p>
|
|
|
|
<p>
|
|
Once these changes are in place, it's necessary to run 'make rpcgen'
|
|
in the src directory to create the .c and .h files required by the
|
|
remote protocol code. This must be done on a Linux host using the
|
|
GLibC rpcgen program. Other rpcgen versions may generate code which
|
|
results in bogus compile time warnings
|
|
</p>
|
|
|
|
|
|
<h2><a name='rpcclient'>Implement the RPC client</a></h2>
|
|
|
|
<p>
|
|
Implementing the RPC client is also relatively mechanical, so refer to
|
|
the exising code and example patch for guidance. The RPC client uses
|
|
the rpcgen generated .h files. The remote method calls go in:
|
|
</p>
|
|
|
|
<p><code>src/remote/remote_internal.c</code></p>
|
|
|
|
<p>Each remote method invocation does the following:</p>
|
|
|
|
<ol class="ordinarylist">
|
|
<li>locks the remote driver;</li>
|
|
<li>sets up the method arguments;</li>
|
|
<li>invokes the remote function;</li>
|
|
<li>checks the return value, if necessary;</li>
|
|
<li>extracts any returned data;</li>
|
|
<li>frees any returned data;</li>
|
|
<li>unlocks the remote driver.</li>
|
|
</ol>
|
|
|
|
<p>
|
|
Once you have created the remote method calls, you have to add fields
|
|
for them to the driver structs for the appropriate remote driver.
|
|
</p>
|
|
|
|
<p class="example">See <a href="api_extension/0005-Step-5-of-8-Implement-the-RPC-client.patch">0005-Step-5-of-8-Implement-the-RPC-client.patch</a></p>
|
|
|
|
<h2><a name="serverdispatch">Implement the server side dispatcher</a></h2>
|
|
|
|
<p>
|
|
Implementing the server side of the remote function calls is simply a
|
|
matter of deserializing the parameters passed in from the remote
|
|
caller and passing them to the corresponding internal API function.
|
|
The server side dispatchers are implemented in:
|
|
</p>
|
|
|
|
<p><code>daemon/remote.c</code></p>
|
|
|
|
<p>Again, this step uses the .h files generated by make rpcgen.</p>
|
|
|
|
<p class="example">See <a href="api_extension/0006-Step-6-of-8-Implement-the-server-side-dispatcher.patch">0006-Step-6-of-8-Implement-the-server-side-dispatcher.patch</a></p>
|
|
|
|
|
|
<h2><a name="driverimpl">Implement the driver methods</a></h2>
|
|
|
|
<p>
|
|
So, after all that, we get to the fun part. All functionality in
|
|
libvirt is implemented inside a driver. Thus, here is where you
|
|
implement whatever functionality you're adding to libvirt. You'll
|
|
either need to add additional files to the src directory or extend
|
|
files that are already there, depending on what functionality you're
|
|
adding.
|
|
</p>
|
|
|
|
<p>
|
|
In the example code, the extension is only an additional two function
|
|
calls in the node device API, so most of the new code is additions to
|
|
existing files. The only new files are there for multi-platform
|
|
implementation convenience, as some of the new code is Linux specific.
|
|
</p>
|
|
|
|
<p>
|
|
The example code is probably uninteresting unless you're concerned
|
|
with libvirt storage, but I've included it here to show how new files
|
|
are added to the build environment.
|
|
</p>
|
|
|
|
<p class="example">See <a href="api_extension/0007-Step-7-of-8-Implement-the-driver-methods.patch">0007-Step-7-of-8-Implement-the-driver-methods.patch</a></p>
|
|
|
|
<h2><a name="virsh">Implement virsh commands</a></h2>
|
|
|
|
<p>
|
|
Once you have the new functionality in place, the easiest way to test
|
|
it and also to provide it to end users is to implement support for it
|
|
in virsh.
|
|
</p>
|
|
|
|
<p>
|
|
A virsh command is composed of a few pieces of code. You need to
|
|
define an array of vshCmdInfo structs for each new command that
|
|
contain the help text and the command description text. You also need
|
|
an array of vshCmdOptDef structs to describe the command options.
|
|
Once you have those pieces of data in place you can write the function
|
|
implementing the virsh command. Finally, you need to add the new
|
|
command to the commands[] array.
|
|
</p>
|
|
|
|
<p class="example">See <a href="api_extension/0008-Step-8-of-8-Add-virsh-support.patch">0008-Step-8-of-8-Add-virsh-support.patch</a></p>
|
|
|
|
<p>Once you have working functionality, run make check and make
|
|
syntax-check before generating patches.</p>
|
|
</body>
|
|
</html>
|