Re: [PATCH v1 0/4] KVM: s390: Permit ESA mode guests in VSIE

Christoph Hellwig
2026-04-06T05:37:30Z

On Wed, Apr 01, 2026 at 12:00:59PM +0200, Janosch Frank wrote:
> KVM is the latest hypervisor in a long list on s390. As such we implemented
> the newest architecture as our starting point and only architecture
> (zArch/64bit).
>
> There are others which don't start their guests from zArch mode but start
> them in ESA and slowly move them through modes until they reach zArch. Those
> other hypervisors often implement emulation for features and devices that
> KVM doesn't support so there's some demand for using them.
>
> Why do we nest those hypervisors under KVM? We have products that work with
> the KVM API so it's easier to nest than to re-write the product to use other
> hypervisors in the first place.

Makes sense. Maybe mention this in the cover letter, and also that
you're not looking into full device model support for older systems?