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434 lines
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434 lines
18 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 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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