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