Wednesday, November 6, 2019

s390x changes in QEMU 4.2

You know the drill: QEMU is entering freeze (this time for 4.2), and there's a post on the s390x changes for the upcoming release.


  • Emulation for  IEP (Instruction Execution Protection), a z14 feature, has been added.
  • A bunch of fixes in the vector instruction emulation and in the fault-handling code.


  • For quite some time now, the code has been implicitly relying on the presence of the 'flic' (floating interrupt controller) KVM device (which had been added in Linux 3.15). Nobody really complained, so we won't try to fix this up and instead make the dependency explicit.
  • The KVM memslot handling was reworked to be actually sane. Unfortunately, this breaks migration of huge KVM guests with more than 8TB of memory from older QEMUs. Migration of guests with less than 8TB continues to work, and there's no planned breakage of migration of >8TB guests starting with 4.2.

CPU models

  • We now know that the gen15a is called 'z15', so reflect this in the cpu model description.
  • The 'qemu' and the 'max' models gained some more features.
  • Under KVM, 'query-machines' will now return the correct default cpu model ('host-s390x-cpu').


  • The usual array of bugfixes, including in SCLP handling and in the s390-ccw bios.