Re: [PATCH v7 23/23] KVM: s390: arm64: Add KVM_S390_ARM64 Kconfig and Makefile

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 03, 2026, Steffen Eiden wrote:
> On Thu, Sep 03, 2026 at 07:43:58AM -0700, Sean Christopherson wrote:
> > And FWIW, while it might seem daunting, from my perspective it's not actually that
> > much churn to do things "right".  Provide KVM_S390_NATIVE, and then have KVM reflect
> > the "weakest" of S390_NATIVE vs. S390_ARCH.  I.e. make KVM=m if either of the "real"
> > KVMs will be a module.  That requires some creative shenanigans, but it's not hard,
> > just weird.
> 
> FYI the first versions of this series had a 3 configs approach very
> similar to yours. 
> 
> IIRC it was not so much the churn we have in (upstream) kernel code but
> more on the distro side and to everyone building the kernel in their
> favourite architecture (s390 :))

How many people are running distro kernels on s390 hardware?  And how many distros
actually change the default KVM settings, e.g. to build KVM as a module instead of
baking it into the kernel?  My guess is "not many" and "almost none", i.e. the
actual impact on downstream users is likely miniscule.

> Suddenly the KVM config changed its behaviour (effectively becoming a noop)

Not really, because "KVM" itself is inaccessible (well, unless someone is hand-
editing .configs or generating them by script, but that's their own fault).
E.g. upgrading to a new kernel will explicitly prompt the user to choose for both
KVM_S390_NATIVE and KVM_S390_ARM64 (or whatever they get called).

> I am starting to think that having no separate config option for arm on
> s390 might be the easisest way. Just KVM and guard both modules behind
> it. It reduces the config space bloat and I do not see a reason why somoeone
> should only compile one KVM module but not the other. kvm-arm64 won't
> load if you do not have the hardware anyways.

Because there may be an unforseen need down the road?  Smushing things together
after the fact is generally easy, pulling things apart without breaking everything
is usually much, much harder.

> But I am not opposed to the x86 approach. Just thinking loud.
> 
> Anyways, I am off for vacation expect a reduced reply frequency from my
> side :) 

Enjoy your holiday!