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

Janosch Frank
2026-04-01T10:01:20Z

On 4/1/26 08:40, Christoph Hellwig wrote:
> On Wed, Apr 01, 2026 at 04:09:11AM +0200, Eric Farman wrote:
>> Hi Hendrik, Janosch, et al,
>>
>> Attached is a small series to allow ESA-mode guests in s390 VSIE.
>> It continues to expect that z/Architecture is the default,
>> unless opted in by userspace.
>>
>> I look forward to your thoughts; thanks!
>
> What is the use case for this? Migrating existin VMs from 31-bit hosts
> to 64-bit hosts?
>

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.