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372 lines
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XML
372 lines
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XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html>
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<html xmlns="http://www.w3.org/1999/xhtml">
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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. Remember that new API consists of any new public
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functions, as well as the addition of flags or extensions of XML used by
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existing functions.
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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="https://libvirt.org/downloads.html">https://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>implement the remote protocol:
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<ol>
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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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</ol>
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</li>
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<li>use new API where appropriate in drivers</li>
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<li>add virsh support</li>
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<li>add common handling for new API</li>
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<li>for each driver that can support the new API:
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<ol>
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<li>add prerequisite support</li>
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<li>fully implement new API</li>
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</ol>
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</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 of one patch per step. That's not to say
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submit patches before you have working functionality--get the whole thing
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working and make sure you're happy with it. Then use git to break the
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changes into pieces so you don't drop a big blob of code on the
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mailing list in one go. Also, you should follow the upstream tree, and
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rebase your series to adapt your patches to work with any other changes
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that were accepted upstream during your development.
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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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<h2><a id='publicapi'>Defining the public API</a></h2>
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<p>The first task is to define the public API. If the new API
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involves an XML extension, you have to enhance the RelaxNG
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schema and document the new elements or attributes:</p>
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<p><code>
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docs/schemas/domaincommon.rng<br/>
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docs/formatdomain.html.in
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</code></p>
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<p>If the API extension involves a new function, you have to add a
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declaration in the public header, and arrange to export the
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function name (symbol) so other programs can link against the
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libvirt library and call the new function:</p>
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<p><code>
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include/libvirt/libvirt-$MODULE.h.in
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src/libvirt_public.syms
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</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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<h2><a id='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 entirely 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-$MODULE.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. Then, update all existing instances of the driver to
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provide a <code>NULL</code> stub for the new function.
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</p>
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<h2><a id='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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virCheckConnectReturn() or virCheckConnectGoto();</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 with
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virCheckReadOnlyGoto();</strong></li>
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<li>SHOULD do basic validation of the parameters that are being
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passed in, using helpers like virCheckNonNullArgGoto();</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-$MODULE.c</code></p>
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<h2><a id='remoteproto'>Implementing the remote protocol</a></h2>
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<p>
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Implementing the remote protocol is essentially a
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straightforward exercise which is probably most easily
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understood by referring to the existing code.
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</p>
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<h3><a id='wireproto'>Defining the wire protocol format</a></h3>
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<p>
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Defining the wire protocol involves making 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 0 or -1 for 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>
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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. This regenerates the
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following files:
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</p>
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<p><code>
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src/remote/remote_daemon_dispatch_stubs.h
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src/remote/remote_daemon_dispatch.h
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src/remote/remote_daemon_dispatch.c
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src/remote/remote_protocol.c
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src/remote/remote_protocol.h
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</code></p>
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<h3><a id='rpcclient'>Implement the RPC client</a></h3>
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<p>
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Implementing the RPC client uses the rpcgen generated .h files.
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The remote method calls go in:
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</p>
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<p><code>src/remote/remote_driver.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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<h3><a id="serverdispatch">Implement the server side dispatcher</a></h3>
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<p>
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Implementing the server side of the remote function call 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>src/remote/remote_daemon_dispatch.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>
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After all three pieces of the remote protocol are complete, and
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the generated files have been updated, it will be necessary to
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update the file:</p>
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<p><code>src/remote_protocol-structs</code></p>
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<p>
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This file should only have new lines added; modifications to
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existing lines probably imply a backwards-incompatible API change.
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</p>
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<h2><a id="internaluseapi">Use the new API internally</a></h2>
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<p>
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Sometimes, a new API serves as a superset of existing API, by
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adding more granularity in what can be managed. When this is
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the case, it makes sense to share a common implementation by
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making the older API become a trivial wrapper around the new
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API, rather than duplicating the common code. This step should
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not introduce any semantic differences for the old API, and is
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not necessary if the new API has no relation to existing API.
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</p>
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<h2><a id="virshuseapi">Expose the new API in virsh</a></h2>
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<p>
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All new API should be manageable from the virsh command line
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shell. This proves that the API is sufficient for the intended
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purpose, and helps to identify whether the proposed API needs
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slight changes for easier usage. However, remember that virsh
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is used to connect to hosts running older versions of libvirtd,
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so new commands should have fallbacks to an older API if
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possible; implementing the virsh hooks at this point makes it
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very easy to test these fallbacks. Also remember to document
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virsh additions.
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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 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. The following files need changes:
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</p>
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<p><code>
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tools/virsh-$MODULE.c<br/>
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tools/virsh.pod
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</code></p>
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<h2><a id="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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<h3><a id="commonimpl">Implement common handling</a></h3>
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<p>
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If the new API is applicable to more than one driver, it may
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make sense to provide some utility routines, or to factor some
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of the work into the dispatcher, to avoid reimplementing the
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same code in every driver. In the example code, this involved
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adding a member to the virDomainDefPtr struct for mapping
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between the XML API addition and the in-memory representation of
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a domain, along with updating all clients to use the new member.
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Up to this point, there have been no changes to existing
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semantics, and the new APIs will fail unless they are used in
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the same way as the older API wrappers.
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</p>
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<h3><a id="drivercode">Implement driver handling</a></h3>
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<p>
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The remaining patches should only touch one driver at a time.
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It is possible to implement all changes for a driver in one
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patch, but for review purposes it may still make sense to break
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things into simpler steps. Here is where the new APIs finally
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start working.
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</p>
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<p>
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It is always a good idea to patch the test driver in addition to the
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target driver, to prove that the API can be used for more than one
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driver.
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</p>
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<p>
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Any cleanups resulting from the changes should be added as separate
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patches at the end of the series.
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</p>
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<p>
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Once you have working functionality, run make check and make
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syntax-check on each patch of the series before submitting
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patches. It may also be worth writing tests for the libvirt-TCK
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testsuite to exercise your new API, although those patches are
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not kept in the libvirt repository.
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</p>
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</body>
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</html>
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