Implementing a new API in Libvirt

This document walks you through the process of implementing a new API in libvirt. Remember that new API consists of any new public functions, as well as the addition of flags or extensions of XML used by existing functions.

Before you begin coding, it is critical that you propose your changes on the libvirt mailing list and get feedback on your ideas to make sure what you're proposing fits with the general direction of the project. Even before doing a proof of concept implementation, send an email giving an overview of the functionality you think should be added to libvirt. Someone may already be working on the feature you want. Also, recognize that everything you write is likely to undergo significant rework as you discuss it with the other developers, so don't wait too long before getting feedback.

Adding a new API to libvirt is not difficult, but there are quite a few steps. This document assumes that you are familiar with C programming and have checked out the libvirt code from the source code repository and successfully built the existing tree. Instructions on how to check out and build the code can be found at:

https://libvirt.org/downloads.html

Once you have a working development environment, the steps to create a new API are:

  1. define the public API
  2. define the internal driver API
  3. implement the public API
  4. implement the remote protocol:
    1. define the wire protocol format
    2. implement the RPC client
    3. implement the server side dispatcher
  5. use new API where appropriate in drivers
  6. add virsh support
  7. add common handling for new API
  8. for each driver that can support the new API:
    1. add prerequisite support
    2. fully implement new API

It is, of course, possible to implement the pieces in any order, but if the development tasks are completed in the order listed, the code will compile after each step. Given the number of changes required, verification after each step is highly recommended.

Submit new code in the form of one patch per step. That's not to say submit patches before you have working functionality--get the whole thing working and make sure you're happy with it. Then use git to break the changes into pieces so you don't drop a big blob of code on the mailing list in one go. Also, you should follow the upstream tree, and rebase your series to adapt your patches to work with any other changes that were accepted upstream during your development.

Don't mix anything else into the patches you submit. The patches should be the minimal changes required to implement the functionality you're adding. If you notice a bug in unrelated code (i.e., code you don't have to touch to implement your API change) during development, create a patch that just addresses that bug and submit it separately.

Defining the public API

The first task is to define the public API. If the new API involves an XML extension, you have to enhance the RelaxNG schema and document the new elements or attributes:

docs/schemas/domaincommon.rng
docs/formatdomain.html.in

If the API extension involves a new function, you have to add a declaration in the public header, and arrange to export the function name (symbol) so other programs can link against the libvirt library and call the new function:

include/libvirt/libvirt-$MODULE.h.in src/libvirt_public.syms

This task is in many ways the most important to get right, since once the API has been committed to the repository, it's libvirt's policy never to change it. Mistakes in the implementation are bugs that you can fix. Make a mistake in the API definition and you're stuck with it, so think carefully about the interface and don't be afraid to rework it as you go through the process of implementing it.

Defining the internal API

Each public API call is associated with a driver, such as a host virtualization driver, a network virtualization driver, a storage virtualization driver, a state driver, or a device monitor. Adding the internal API is ordinarily a matter of adding a new member to the struct representing one of these drivers.

Of course, it's possible that the new API will involve the creation of an entirely new driver type, in which case the changes will include the creation of a new struct type to represent the new driver type.

The driver structs are defined in:

src/driver-$MODULE.h

To define the internal API, first typedef the driver function prototype and then add a new field for it to the relevant driver struct. Then, update all existing instances of the driver to provide a NULL stub for the new function.

Implementing the public API

Implementing the public API is largely a formality in which we wire up public API to the internal driver API. The public API implementation takes care of some basic validity checks before passing control to the driver implementation. In RFC 2119 vocabulary, this function:

  1. SHOULD log a message with VIR_DEBUG() indicating that it is being called and its parameters;
  2. MUST call virResetLastError();
  3. SHOULD confirm that the connection is valid with virCheckConnectReturn() or virCheckConnectGoto();
  4. SECURITY: If the API requires a connection with write privileges, MUST confirm that the connection flags do not indicate that the connection is read-only with virCheckReadOnlyGoto();
  5. SHOULD do basic validation of the parameters that are being passed in, using helpers like virCheckNonNullArgGoto();
  6. MUST confirm that the driver for this connection exists and that it implements this function;
  7. MUST call the internal API;
  8. SHOULD log a message with VIR_DEBUG() indicating that it is returning, its return value, and status.
  9. MUST return status to the caller.

The public API calls are implemented in:

src/libvirt-$MODULE.c

Implementing the remote protocol

Implementing the remote protocol is essentially a straightforward exercise which is probably most easily understood by referring to the existing code.

Defining the wire protocol format

Defining the wire protocol involves making additions to:

src/remote/remote_protocol.x

First, create two new structs for each new function that you're adding to the API. One struct describes the parameters to be passed to the remote function, and a second struct describes the value returned by the remote function. The one exception to this rule is that functions that return only 0 or -1 for status do not require a struct for returned data.

Second, add values to the remote_procedure enum for each new function added to the API.

Once these changes are in place, it's necessary to run 'make rpcgen' in the src directory to create the .c and .h files required by the remote protocol code. This must be done on a Linux host using the GLibC rpcgen program. Other rpcgen versions may generate code which results in bogus compile time warnings. This regenerates the following files:

src/remote/remote_daemon_dispatch_stubs.h src/remote/remote_daemon_dispatch.h src/remote/remote_daemon_dispatch.c src/remote/remote_protocol.c src/remote/remote_protocol.h

Implement the RPC client

Implementing the uses the rpcgen generated .h files. The remote method calls go in:

src/remote/remote_driver.c

Each remote method invocation does the following:

  1. locks the remote driver;
  2. sets up the method arguments;
  3. invokes the remote function;
  4. checks the return value, if necessary;
  5. extracts any returned data;
  6. frees any returned data;
  7. unlocks the remote driver.

Implement the server side dispatcher

Implementing the server side of the remote function call is simply a matter of deserializing the parameters passed in from the remote caller and passing them to the corresponding internal API function. The server side dispatchers are implemented in:

src/remote/daemon_dispatch.c

Again, this step uses the .h files generated by make rpcgen.

After all three pieces of the remote protocol are complete, and the generated files have been updated, it will be necessary to update the file:

src/remote_protocol-structs

This file should only have new lines added; modifications to existing lines probably imply a backwards-incompatible API change.

Use the new API internally

Sometimes, a new API serves as a superset of existing API, by adding more granularity in what can be managed. When this is the case, it makes sense to share a common implementation by making the older API become a trivial wrapper around the new API, rather than duplicating the common code. This step should not introduce any semantic differences for the old API, and is not necessary if the new API has no relation to existing API.

Expose the new API in virsh

All new API should be manageable from the virsh command line shell. This proves that the API is sufficient for the intended purpose, and helps to identify whether the proposed API needs slight changes for easier usage. However, remember that virsh is used to connect to hosts running older versions of libvirtd, so new commands should have fallbacks to an older API if possible; implementing the virsh hooks at this point makes it very easy to test these fallbacks. Also remember to document virsh additions.

A virsh command is composed of a few pieces of code. You need to define an array of vshCmdInfo structs for each new command that contain the help text and the command description text. You also need an array of vshCmdOptDef structs to describe the command options. Once you have those pieces in place you can write the function implementing the virsh command. Finally, you need to add the new command to the commands[] array. The following files need changes:

tools/virsh-$MODULE.c
tools/virsh.pod

Implement the driver methods

So, after all that, we get to the fun part. All functionality in libvirt is implemented inside a driver. Thus, here is where you implement whatever functionality you're adding to libvirt. You'll either need to add additional files to the src directory or extend files that are already there, depending on what functionality you're adding.

Implement common handling

If the new API is applicable to more than one driver, it may make sense to provide some utility routines, or to factor some of the work into the dispatcher, to avoid reimplementing the same code in every driver. In the example code, this involved adding a member to the virDomainDefPtr struct for mapping between the XML API addition and the in-memory representation of a domain, along with updating all clients to use the new member. Up to this point, there have been no changes to existing semantics, and the new APIs will fail unless they are used in the same way as the older API wrappers.

Implement driver handling

The remaining patches should only touch one driver at a time. It is possible to implement all changes for a driver in one patch, but for review purposes it may still make sense to break things into simpler steps. Here is where the new APIs finally start working.

It is always a good idea to patch the test driver in addition to the target driver, to prove that the API can be used for more than one driver.

Any cleanups resulting from the changes should be added as separate patches at the end of the series.

Once you have working functionality, run make check and make syntax-check on each patch of the series before submitting patches. It may also be worth writing tests for the libvirt-TCK testsuite to exercise your new API, although those patches are not kept in the libvirt repository.