Wednesday, April 8, 2020

s390x changes in QEMU 5.0

QEMU is currently in hardfreeze, with the 5.0 release expected at the end of the month. Here's a quick list of some notable s390x changes.

  • You can finally enable Adapter Interrupt Suppression in the cpu model (ais=on) when running under KVM. This had been working under TCG for some time now, but KVM was missing an interface that was provided later -- and we finally actually check for that interface in QEMU. This is mostly interesting for PCI.
  • QEMU had been silently fixing odd memory sizes to something that can be reported via SCLP for some time. Silently changing user input is probably not such a good idea; compat machines will continue to do so to enable migration from old QEMUs for machines with odd sizes, but will print a warning now. If you have such an old machine (and you can modify it), it might be a good idea to either specify the memory size it gets rounded to or to switch to the 5.0 machine type, where memory sizes can be more finegrained due to the removal of support for memory hotplug. We may want to get rid of the code doing the fixup at some time in the future.
  • QEMU now properly performs the whole set of initial, clear, and normal cpu reset.
  • And the usual fixes, cleanups, and improvements.
For 5.1, expect more changes; support for protected virtualization will be a big item.

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