Friday, December 20, 2019

A 2019 recap (and a bit of an outlook)

The holiday season for 2019 will soon be upon us, so I decided to do a quick recap of what I consider the highlights for this year, from my perspective, regarding s390x virtualization, and the wider ecosystem.


I attended the following conferences last year.

Linux Plumbers Conference

LPC 2019 was held in Lisbon, Portugal, on September 9-11. Of particular interest for me was the VFIO/IOMMU/PCI microconference. I talked a bit about cross-architecture considerations (and learned about some quirks on other architectures as well); the rest of the topics, while not currently concerning my work directly, were nevertheless helpful to move things forward. As usual on conferences, the hallway track is probably the most important one; met some new folks, saw others once again, and talked more about s390 I/O stuff than I anticipated. I can recommend this conference for meeting people to talk to about (not only) deeply technical things.

KVM Forum

KVM Forum 2019 was held in Lyon, France, on October 30 - November 1. As usual, a great place to meat people and have discussions in the hallway, e.g. about vfio migration. No talk from me this year, but an assortment of interesting topics presented by others; I contributed to an article on ( Of note from an s390x perspective were the talks about protected virtualization and nested testing mentioned in the article, and also the presentation on running kvm unit tests beyond KVM.

s390x changes in QEMU and elsewhere

There's a new machine (z15) on the horizon, but support for older things has been introduced or enhanced as well.


Lots of work has gone into tcg to emulate the vector instructions introduced with z13. Distributions are slowly switching to compiling for z13, which means gcc is generating vector instructions. As of QEMU 4.2, you should be able to boot recent distributions under tcg once again.


vfio-ccw now has gained support for sending HALT SUBCHANNEL and CLEAR SUBCHANNEL to the real device; this is useful e.g. for error handling, when you want to make sure an operation is really terminated at the device. Also, it is now possible to boot from a DASD attached via vfio-ccw.

vfio-ap has seen some improvements as well, including support for hotplugging the matrix device and for interrupts.

Guest side

A big change on the guest side of things was support for protected virtualization (also see the talk given at KVM Forum). This is a bit like AMD's SEV, but (of course) different. Current Linux kernels should be ready to run as a protected guest; host side support is still in progress (see below).

Other developments of interest

mdev, mdev everywhere

There has been a lot of activity around mediated devices this year. They have been successfully used in various places in the past (GPUs, vfio-ccw, vfio-ap, ...). A new development is trying to push parts of it into userspace ('muser', see the talk at KVM Forum). An attempt was made to make use of the mediating part without the vfio part, but that was met with resistance. Ideas for aggregation are still being explored.

In order to manage and persist mdev devices, we introduced the mdevctl tool, which is currently included in at least Fedora and Debian.

vfio migration

Efforts to introduce generic migration support for vfio (or at least in the first shot, for pci) are still ongoing. Current concerns mostly cycle around dirty page tracking. It might make sense to take a stab at wiring up vfio-ccw once the interface is stable.

What's up next?

While there probably will be some not-yet-expected developments next year, some things are bound to come around in 2020.

Protected virtualization

Patch sets for KVM and QEMU to support protected virtualization on s390 have already been posted this year; expect new versions of the patch sets to show up in 2020 (and hopefully make their way into the kernel respectively QEMU).


Patches to support detecting path status changes and relaying them to the guest have already been posted; expect an updated version to make its way into the kernel and QEMU in 2020. Also likely: further cleanups and bugfixes, and probably some kind of testing support, e.g. via kvm unit tests. Migration support also might be on that list.

virtio-fs support on s390x

Instead of using virtio-9p, virtio-fs is a much better way to share files between host and guest; expect support on s390x to become available once sharing of fds in QEMU without numa becomes possible. Shared memory regions on s390x (for DAX support) still need discussion, however.