Wednesday, November 14, 2018

s390x changes in QEMU 3.1

QEMU is now in the -rc phase for 3.1, with a release expected in early/mid December, and, as usual, this is a good time to summarize the s390x changes for that release.

CPU models

  • s390x now supports the 'max' cpu model as well (which somehow had been forgotten...) When using KVM, this behaves like the 'host' model; when using TCG, this is the 'qemu' model plus some additional, experimental features. Note that this is neither static nor migration-safe.

Devices

  • Support for vfio-ap has been added. That allows to pass crypto cards on the AP bus to the guest. Support for this has been merged into the Linux kernel with 4.20. As this is a rather large feature, I plan to do a separate writeup for this.

KVM

  • Support for enabling huge page backing has been added. This requires a host kernel of version 4.19 or higher. Note that this is only available for the s390-ccw-virtio-3.1 or later machines (due to compat handling), and that it is as of writing this incompatible with nested virtualization (which should change in the future.)
  • Support for the etoken facility (spectre mitigation) has been added. This, as well, needs a host kernel of version 4.19 or higher.

TCG

  • Support for instruction flags and AFP registers has been added.

Miscellaneous

  • The deprecated 's390-squash-mcss' option has been removed.
  • And the usual fixes, cleanups and improvements.

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