Re: [PATCH v2 2/2] Documentation/process: Add a maintainer handbook for KVM x86

[Oliver Upton]

On Thu, Mar 09, 2023 at 09:25:54AM -0800, Sean Christopherson wrote:
> On Thu, Mar 09, 2023, Oliver Upton wrote:
> > On Thu, Mar 09, 2023 at 09:37:45AM +0700, Bagas Sanjaya wrote:
> > > On Wed, Mar 08, 2023 at 05:03:36PM -0800, Sean Christopherson wrote:
> > > > +As a general guideline, use ``kvm-x86/next`` even if a patch/series touches
> > > > +multiple architectures, i.e. isn't strictly scoped to x86.  Using any of the
> > > > +branches from the main KVM tree is usually a less good option as they likely
> > > > +won't have many, if any, changes for the next release, i.e. using the main KVM
> > > > +tree as a base is more likely to yield conflicts.  And if there are non-trivial
> > > > +conflicts with multiple architectures, coordination between maintainers will be
> > > > +required no matter what base is used.  Note, this is far from a hard rule, i.e.
> > > > +use a different base for multi-arch series if that makes the most sense.
> > 
> > I don't think this is the best way to coordinate with other architectures.
> > Regardless of whether you intended this to be prescriptive, I'm worried most
> > folks will follow along and just base patches on kvm-x86/next anyway.
> 
> Probably, but for the target audience (KVM x86 contributors), that's likely the
> least awful base 99% of the time.

Sorry, I follow this reasoning at all.

If folks are aiming to make a multi-arch contribution then the architecture
they regularly contribute to has absolutely zero relevance on the series
itself.

> > It'd be easier to just have multi-arch series use a stable base (i.e. a
> > release candidate) by default. That'd avoid the undesirable case where merging
> > a shared branch brings with it some random point in another arch's /next
> > history.
> 
> You're conflating the base of the patch series with the branch it is applied to.

We cannot pretend the two are in no way related. The dependencies of a series
are not obvious when based on the /next branch of any one architecture.

> I'm most definitely not proposing that multi-arch series from x86 contributors
> always be routed through kvm-x86.  It's ultimately the responsibility of the
> maintainers, not the contributors, to avoid funky merges and histories.

Right, but contributors looking to make changes across architectures share
some of the burden of cross-arch coordination as well. Basing patches off of
a random commit not in Linus' tree does not match at least the arm64
workflow.

> If a
> series warrants a dedicated topic branch, then we need to create said topic branch
> off a stable, common base, irrespective of what the contributor based their patches
> on.

The lowest friction way to coordinate such things is to start off with a
common base and go from there. If there is a compelling argument for doing
things differently in the context of one series then let's talk about it on
the list.

> If a series from an x86 contributor applies cleanly on kvm-x86/next but not on
> -rc2 (or whatever), then the reverse would also likely be true (if the contributor
> used -rc2 as the base).

This can be addressed in a merge resolution, thereby offloading the
responsibility to the maintainer.

> In other words, for series with non-trivial modifications
> to other architectures and/or common KVM code, IMO the base used for the _initial_
> posting doesn't matter all that much for us maintainers since such series will
> likely require additional attention no matter what base is used.

In all likelihood, sure the series will be respun. But, you're offloading the
responsibility to ask for a sane base on other arch maintainers which I'm not
cool with.

> On the flip side, the vast majority of "multi-arch" series in KVM tend to be focused
> on a single architecture, with only incidental contact to other architectures and/or
> common KVM code.  Those types of series will likely be routed through their "target"
> arch tree, and so for x86, using kvm-x86/next as the base is preferrable.

With long term aspirations to share more code between architectures (e.g.
common MMU) I believe we'll see more changes that have meaningful interaction
with all architecutures.

> My goal with suggesting/prescribing kvm-x86/next to contributors is to make the
> easy things easy.  On my end, that means having _predictable_ submissions and
> minimizing the number of avoidable conflicts.  For contributors, that means having
> a very simple rule/guideline.  "Use kvm-x86/next unless you know better" satisfies
> all those conditions.

I believe "Use a release candidate unless you know better" for multi-arch
changes is just as simple. Better yet, it clues in contributors as to how
changes are coordinated across architectures and might help them know better
next time around.

> > If a different approach makes sense for a particular series then we can
> > discuss it on the list and arrive at something agreeable for all parties
> > involved.
> > 
> > > That means patches that primarily kvm ARM changes should be based on
> > > kvm-x86/next, right?
> > 
> > No, don't do that.
> 
> +<infinity symbol>
> 
> This doc is specifically for KVM x86.

You've also made some suggestions about cross-arch development that do not fit
the development model of other architectures. I have no desire to nitpick
about the x86 process but want the multiarch language to actually set folks up
for success working outside of the KVM/x86 tree.

-- 
Thanks,
Oliver