Features/Documentation

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QEMU's documentation needs some reorganization.

Ultimately there should be five manuals:

QEMU User-mode Emulation User's Guide (docs/user, Features/Documentation/user)
currently in qemu-doc.texi
QEMU System Emulation User's Guide (docs/system, Features/Documentation/system)
Category:User documentation
the largest part of qemu-doc.texi
also covers qemu-img, qemu-io
docs/ (not subdirectories)
QEMU System Emulation Management and Interoperability Guide (docs/interop, Features/Documentation/interop)
QEMU System Emulation Guest Hardware Specifications (docs/specs, Features/Documentation/specs)
QEMU Developer's Guide (docs/devel, Features/Documentation/devel)
Category:Developer documentation
docs/devel/
the implementation notes in qemu-doc.texi
tcg/README
doc comments in the source code
automatically generated docs for Python classes in qemu-iotests and scripts/qmp
not to be shipped to end users (unlike the other manuals)

Choices

Based on experience from the Linux kernel, QEMU's docs pipeline is going to be based on Sphinx (sample developer doc, sample user doc. Sphinx is extensible and it is easy to add new input formats and input directives.

Currently, QEMU documentation is written in a mix of Texinfo and text files roughly based on Markdown. Sphinx's native format is reStructuredText (rST). The plan is to (eventually) convert everything to rST format.

Currently, QEMU doc comments have never been actually used together with a actual documentation generator in mind, but they are roughly based on gtk-doc syntax. We will probably end up keeping a copy of the Linux kernel's kernel-doc script. kernel-doc's main supported backend is rST. There is a GSoC project to look at generating API documentation from our doc comments.

Pre-Sphinx status

(This concentrates on the documentation that's installed by 'make install', because anything else is for developers only, making it lower priority to clean up and more amenable to messing around with anyway.)

As of QEMU 4.0 (early 2019) we install:

  • in $DESTDIR/usr/local/share/doc/qemu:
qemu-doc.html
qemu-ga-ref.html
qemu-qmp-ref.html
qemu-doc.txt
qemu-ga-ref.txt
qemu-qmp-ref.txt
interop/  (the only Sphinx manual we currently ship)
  • in $DESTDIR/usr/local/share/man/:
man1/qemu.1
man1/qemu-img.1
man1/virtfs-proxy-helper.1
man1/qemu-trace-stap.1
man8/qemu-ga.8
man8/qemu-nbd.8
man7/qemu-block-drivers.7
man7/qemu-cpu-models.7
man7/qemu-ga-ref.7
man7/qemu-qmp-ref.7

(possibly I missed one or two files, because what we install depends on configure options and maybe my test build-n-install didn't build everything we might ship.)

This documentation is generated from a mix of:

  • hand-written texinfo:
qemu-doc.texi (a top level file with both content and @include directives)
qemu-deprecated.texi
qemu-ga.texi
qemu-img.texi
qemu-nbd.texi
qemu-option-trace.texi
qemu-tech.texi
docs/security.texi
docs/qemu-cpu-models.texi
docs/qemu-block-drivers.texi
docs/interop/qemu-ga-ref.texi
docs/interop/qemu-qmp-ref.texi
fsdev/virtfs-proxy-helper.texi
scripts/qemu-trace-stap.texi
  • texinfo sections extracted from *.hx files by hxtool
  • texinfo autogenerated from json by qapi-gen.py

(The .html and .txt files are generated from the texinfo directly; the manpages via texi2pod and pod2man, which effectively create the manpages from marked-up subsqections of the input texi.)

Intended final status

We eventually want to have:

  • in $DESTDIR/usr/local/share/doc/qemu:
interop/ sphinx manual
system/ sphinx manual (system emulation user's guide)
user/ sphinx manual (user-mode emulation user's guide)
specs/ sphinx manual (guest hardware etc specs)
  • in $DESTDIR/usr/local/share/man/:
the same set of manpages we have currently
  • not actually installed:
devel/ sphinx manual

Sphinx supports a "plain text" output format, so we could create a "one big text file" format output for each of our four installed manuals. However there doesn't seem to be much support for retaining a plain-text version of the documentation, so we should just ship the HTML version.

Transition plan

This is based on the plan proposed in this email: https://lists.gnu.org/archive/html/qemu-devel/2019-05/msg04932.html

0: API documentation generation

Work on generating API documentation can be done independently and is not addressed here, because that will just go into the devel/ manual, and doesn't interact at all with the user-facing manuals.

1: qemu-ga-ref and qemu-qmp-ref

qemu-ga-ref and qemu-qmp-ref will become part of the interop/ manual. This requires:

  • adding rst-generation support to qapi-gen.py
  • converting the hand-written texinfo parts to rst
  • listing those new .rst files in docs/interop/index.rst
  • makefile runes to create manpages via the 'man' sphinx builder from docs/interop/qemu-ga-ref.rst and qemu-qmp-ref.rst
  • there is a tests/qapi-schema/doc-good.test.texi which I think is acting as a test of the texi doc generation; this should be converted to test the rst doc generation instead
  • delete all the old texinfo build machinery, install runes, etc

We could do this as its own patch series which would end up with a result that would be fine to ship, even if we don't get any of the other manuals converted for a particular QEMU release. It's the easiest part because these docs already have an obvious home in a Sphinx manual we're already shipping.

For the "create manpage" part, Sphinx supports a "manpage" builder, which I have not yet investigated closely but which I'm hoping will do what we need. It works by specifying a list of "this .rst becomes this manpage in this manpage-section", so we can for instance have the qemu-ga-ref and qemu-qmp-ref be subsections of the HTML interop/ manual but also pulled out into their own manpages. (It is also possible to make sections of a .rst file be output only for particular builders, but there seem to be some limitations on it -- notably it filters the output but it doesn't affect things like tables of contents for formats which have them. I am hoping we can do what we want without it.)

2: Move developer docs into devel manual (DONE)

Any parts of the texi docs which are really "developer docs" should be moved into devel/

This is easy because we don't need to maintain a very coherent manual for devel/, we can just have it be a bucket of random documents for the moment. In particular:

  • the qemu-tech.texi "translator internals" section should be moved (I suspect it's a bit out of date so we should add a warning note at the top of the new file about that)
  • I think we should just delete the "QEMU compared to other emulators" section of qemu-tech.texi (and the accompanying bibliography of URLs) since it hasn't been updated in several years and it's not really documenting QEMU. This sort of general-information stuff would live better on the wiki if we care about it at all.
  • I don't think there's anything else that's really developer documentation, but I haven't looked too closely

A patchset doing this has been sent to the list and was applied in time for QEMU 4.1.

3: Convert things which are mostly standalone manpages

Round about here things start to get trickier because we are looking at things which are part of the big fat qemu-doc.html. The easiest next pieces to pull out are the ones which are standalone manpages which we're also just using @include to stuff into qemu-doc.html:

  • qemu-ga.texi
  • qemu-nbd.texi
  • qemu-img.texi
  • qemu-cpu-models.texi
  • qemu-block-drivers.texi
  • virtfs-proxy-helper.texi
  • scripts/qemu-trace-stap.texi

We can:

  • convert each of these to a .rst file in the right sphinx manual (probably mostly system/)
  • generate the manpage output using sphinx as we did for qemu-ga-ref etc
  • just drop the @include from qemu-doc.texi

qemu-img.texi here will be the tricky one as it uses hxtool to generate the texi for the command line option descriptions. I'm not sure how best to handle hxtool, because it is really just a mechanism for slicing the right fragments out of the input file. Perhaps we need to add SRST/ERST directives to match the STEXI/ETEXI ones we have at the moment.

This will result in a slightly "bucket of distinct docs" feel to the system/ manual at this point, but these files are all basically mostly self-contained, so it wouldn't be too terrible to ship a QEMU in this state.

A patch that does this for qemu-ga.texi has been sent to the list: https://patchwork.ozlabs.org/patch/1117784/ and is ready to be applied once QEMU 4.1 has been released.

4: Convert everything else

At this point what we have left is:

  • qemu-doc.texi itself
  • qemu-deprecated.texi
  • docs/security.texi
  • qemu-option-trace.texi
  • the leftovers from qemu-tech.texi
  • the stuff generated via hxtool from hmp-commands-info.hx, hmp-commands.hx and qemu-options.hx

and we pretty much need to:

  • convert all that over to rst in the system/ and user/ manuals (perhaps sometimes interop/)
  • generate the qemu.1 manpage
  • delete all the leftover machinery

We could do at least some of this in chunks, but if we had to release QEMU midway through this part of the transition we'd have a very weird setup where half the core-emulator-docs were in the old html file and the other half were in the new style manuals, so we should definitely try to avoid doing that.