Re: [PATCH 2/2] Documentation/process: Add a maintainer handbook for KVM x86

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 22, 2023, Maciej S. Szmigiero wrote:
> On 17.02.2023 23:54, Sean Christopherson wrote:
> > +SDM and APM References
> > +~~~~~~~~~~~~~~~~~~~~~~
> > +Much of KVM's code base is directly tied to architectural behavior defined in
> > +Intel's Software Development Manual (SDM) and AMD's Architecture Programmer’s
> > +Manual (APM).  Use of "Intel's SDM" and "AMD's APM", or even just "SDM" or
> > +"APM", without additional context is a-ok.
> > +
> > +Do not reference specific sections, tables, figures, etc. by number, especially
> > +not in comments.  Instead, copy-paste the relevant snippet (if warranted), and
> > +reference sections/tables/figures by name.
> 
> This says do "copy-paste the relevant snippet"...
> 
> > The layouts of the SDM and APM are
> > +constantly changing, and so the numbers/labels aren't stable/consistent.
> > +
> > +Generally speaking, do not copy-paste SDM or APM snippets into
> > comments.
> 
> ...but this seems to say "don't do that".

Yeah, that didn't come out right.

> More specific guidance would probably help here.

Is this better?

  Do not reference specific sections, tables, figures, etc. by number, especially
  not in comments.  Instead, if necessary (see below), copy-paste the relevant
  snippet and reference sections/tables/figures by name.  The layouts of the SDM
  and APM are constantly changing, and so the numbers/labels aren't stable.
  
  Generally speaking, do not explicitly reference or copy-paste from the SDM or
  APM in comments.  With few exceptions, KVM *must* honor architectural behavior,
  therefore it's implied that KVM behavior is emulating SDM and/or APM behavior.
  Note, referencing the SDM/APM in changelogs to justify the change and provide
  context is perfectly ok and encouraged.

> > +If a patch touches multiple topics, traverse up the conceptual tree to find the
> > +first common parent (which is often simply ``x86``).  When in doubt,
> > +``git log path/to/file`` should provide a reasonable hint.
> > +
> > +New topics do occasionally pop up, but please start an on-list discussion if
> > +you want to propose introducing a new topic, i.e. don't go rogue.
> > +
> > +Do not use file names or complete file paths as the subject/shortlog prefix.
> 
> Do we strictly obey the "75 characters max" rule for the subject/shortlog
> or do we prefer to be more flexible here if it results in a more
> descriptive patch subject?

I prefer to be a little flexible, I'll expand this section to clarify that.

> (...)
> > +Testing
> > +-------
> > +At a bare minimum, *all* patches in a series must build cleanly for KVM_INTEL=m
> > +KVM_AMD=m, and KVM_WERROR=y.  Building every possible combination of Kconfigs
> > +isn't feasible, but the more the merrier.  KVM_SMM, KVM_XEN, PROVE_LOCKING, and
> > +X86_64 are particularly interesting knobs to turn.
> > +
> > +Running KVM selftests and KVM-unit-tests is also mandatory (and stating the
> > +obvious, the tests need to pass).
> 
> I would add an exception here from mandatory testing for changes that
> obviously have negligible probability of affecting runtime behavior.
> 
> For example: patches that modify just code comments or documentation.

Agreed, will add.

Regarding documentation, I think I'll also add a requirement of 'make htmldocs'
without warnings for non-trivial docs changes.  It's all too easy to write buggy
ReST "code" that looks correct as raw text, e.g. the whole double-colon thing.

> > When possible and relevant, testing on both
> > +Intel and AMD is strongly preferred.  Booting an actual VM is encouraged, but
> > +not mandatory.
> > +
> > +For changes that touch KVM's shadow paging code, running with TDP (EPT/NPT)
> > +disabled is mandatory.  For changes that affect common KVM MMU code, running
> > +with TDP disabled is strongly encouraged.  For all other changes, if the code
> > +being modified depends on and/or interacts with a module param, testing with
> > +the relevant settings is mandatory.
> > +
> > +Note, KVM selftests and KVM-unit-tests do have known failures.  If you suspect
> > +a failure is not due to your changes, verify that the *exact same* failure
> > +occurs with and without your changes.
> > +
> > +If you can't fully test a change, e.g. due to lack of hardware, clearly state
> > +what level of testing you were able to do, e.g. in the cover letter.
> > +
> (...)
> 
> Thanks for preparing such a detailed handbook Sean.
> 
> However, having so many rules might seem intimidating for newcomers, and
> deter them from contributing out of fear that they'll be screamed at for
> accidentally breaking some obscure rule.
> 
> That's especially true for unpaid volunteers that might become
> professional kernel developers one day if their learning curve isn't
> made too steep.
> 
> Maybe have a paragraph or two that, despite all these rules, KVM x86
> strives to be a welcome community, encouraging newcomers and understanding
> their beginner mistakes (or so)?

I like that idea a lot, I'll add a section at the very top.

Thanks much!