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The rule generating the HTML docs passing the --html flag to xsltproc. This makes it use the legacy HTML parser, which either ignores or tries to fix all sorts of broken XML tags. There's no reason why we should be writing broken XML in the first place, so removing --html and adding the XHTML doctype to all files forces us to create good XML. This adds the XHTML doc type and fixes many, many XML tag problems it exposes. Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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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. It uses as an example the addition of an API for
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separating maximum from current vcpu usage of a domain, over
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the course of a fifteen-patch series.
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Remember that new API consists of any new public functions, as
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well as the addition of flags or extensions of XML used by
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existing functions. The example in this document adds both new
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functions and an XML extension. Not all libvirt API additions
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require quite as many patches.
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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. In the vcpu example
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below, list feedback was first requested
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<a href="https://www.redhat.com/archives/libvir-list/2010-September/msg00423.html">here</a>
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and resulted in several rounds of improvements before coding
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began. In turn, this example is slightly rearranged from the actual
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order of the commits.
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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>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 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 in one go.
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Also, you should follow the upstream tree, and rebase your
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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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<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. 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/domain.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.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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<p class="example">See <a href="api_extension/0001-add-to-xml.patch">0001-add-to-xml.patch</a>
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and <a href="api_extension/0002-add-new-public-API.patch">0002-add-new-public-API.patch</a>
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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 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.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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<p class="example">See <a href="api_extension/0003-define-internal-driver-API.patch">0003-define-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/0004-implement-the-public-APIs.patch">0004-implement-the-public-APIs.patch</a></p>
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<h2><a name='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 and the example
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patch. It involves several related changes, including the
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regeneration of derived files, with further details below.
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</p>
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<p class="example">See <a href="api_extension/0005-implement-the-remote-protocol.patch">0005-implement-the-remote-protocol.patch</a></p>
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<h3><a name='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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daemon/remote_dispatch_args.h
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daemon/remote_dispatch_prototypes.h
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daemon/remote_dispatch_table.h
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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 name='rpcclient'>Implement the RPC client</a></h3>
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<p>
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Implementing the uses the rpcgen generated .h files. The remote
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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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<h3><a name="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>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>
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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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<p class="example">See <a href="api_extension/0005-implement-the-remote-protocol.patch">0005-implement-the-remote-protocol.patch</a></p>
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<h2><a name="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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<p class="example">See <a href="api_extension/0006-make-old-API-trivially-wrap-to-new-API.patch">0006-make-old-API-trivially-wrap-to-new-API.patch</a></p>
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<h2><a name="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.c<br/>
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tools/virsh.pod
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</code></p>
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<p class="example">See <a href="api_extension/0007-add-virsh-support.patch">0007-add-virsh-support.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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<h3><a name="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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<p class="example">See <a href="api_extension/0008-support-new-xml.patch">0008-support-new-xml.patch</a></p>
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<h3><a name="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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In the example patches, three separate drivers are supported:
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test, qemu, and xen. It is always a good idea to patch the test
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driver in addition to the target driver, to prove that the API
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can be used for more than one driver. The example updates the
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test driver in one patch:
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</p>
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<p class="example">See <a href="api_extension/0009-support-all-flags-in-test-driver.patch">0009-support-all-flags-in-test-driver.patch</a></p>
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<p>
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The qemu changes were easier to split into two phases, one for
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updating the mapping between the new XML and the hypervisor
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command line arguments, and one for supporting all possible
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flags of the new API:
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</p>
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<p class="example">See <a href="api_extension/0010-improve-vcpu-support-in-qemu-command-line.patch">0010-improve-vcpu-support-in-qemu-command-line.patch</a>
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and <a href="api_extension/0011-complete-vcpu-support-in-qemu-driver.patch">0011-complete-vcpu-support-in-qemu-driver.patch</a></p>
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<p>
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Finally, the example breaks the xen driver changes across four
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patches. One maps the XML changes to the hypervisor command,
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the next two are independently implementing the getter and
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setter APIs, and the last one provides cleanup of code that was
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rendered dead by the new API.
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</p>
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<p class="example">See <a href="api_extension/0012-improve-vcpu-support-in-xen-command-line.patch">0012-improve-vcpu-support-in-xen-command-line.patch</a>,
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<a href="api_extension/0013-improve-getting-xen-vcpu-counts.patch">0013-improve-getting-xen-vcpu-counts.patch</a>,
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<a href="api_extension/0014-improve-setting-xen-vcpu-counts.patch">0014-improve-setting-xen-vcpu-counts.patch</a>,
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and <a href="api_extension/0015-remove-dead-xen-code.patch">0015-remove-dead-xen-code.patch</a></p>
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<p>
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The exact details of the example code are probably uninteresting
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unless you're concerned with virtual cpu management.
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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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