Tuesday, March 12, 2019

s390x changes in QEMU 4.0

QEMU is now entering softfreeze for the 4.0 release (expected in April), so here is the usual summary of s390x changes in that release.

CPU Models

  • A cpu model for the z14 GA 2 has been added. Currently, no new features have been added.
  • The cpu model for z14 now does, however, include the multiple epoch and PTFF enhancement features per default.
  • The 'qemu' cpu model now includes the zPCI feature per default. No more prerequisites are needed for pci support (see below).

Devices


  • QEMU for s390x is now always built with pci support. If we want to provide backwards compatibility,  we cannot simply disable pci (we need the s390 pci host bus); it is easier to simply make pci mandatory. Note that disabling pci was never supported by the normal build system anyway.
  • zPCI devices have gained support for instruction counters (on a Linux guest, these are exposed through /sys/kernel/debug/pci/<function>/statistics).
  • zPCI devices always lacked support for migrating their s390-specific state (not implemented...); if you tried to migrate a guest with a virtio-pci device on s390x, odd things might happen. To avoid surprises, the 'zpci' devices are now explicitly marked as unmigratable. (Support for migration will likely be added in the future.)
  • Hot(un)plug of the vfio-ap matrix device is now supported.
  • Adding a vfio-ap matrix device no longer inhibits usage of a memory ballooner: Memory usage by vfio-ap does not clash with the concept of a memory balloon.

TCG

  • Support for the floating-point extension facility has been added.
  • The first part of support for z13 vector instructions has been added (vector support instructions). Expect support for the remaining vector instructions in the next release; it should support enough of the instructions introduced with z13 to be able to run a distribution built for that cpu. 

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