Tuesday, November 24, 2020

s390x changes in QEMU 5.2

 As, once again, a new QEMU release is around the corner, the time has come to list some s390x changes in there.

  • TCG has gained emulation support for some additional instructions that had been introduced with the z14. More enhancements needed to be able to run distributions built for the z14 will likely come in the future.
  • When running under KVM, QEMU now supports the diagnose 0x318 instruction. This can be used to set some diagnostic information (such as the operating system), which may be helpful when servicing the hardware. With this comes support for extended SCCBs; this is needed as the facility indication for diag318 encroaches into the control block used for reporting CPU information. A guest needs support for extended SCCBs to be able to see information for all CPUs if diag318 support is provided.
  • You can now use virtiofs on s390x, thanks to some endianness fixes, and a vhost-user-fs-ccw device has been added.
  • Up to now, both fully emulated PCI functions and PCI functions passed via vfio-pci reported the same values when the guest issued CLP instructions. However, the passed through functions may use different values for things such as the supported DMA range. If the host kernel supplies the respective capabilities for the vfio-pci device, QEMU can now provide the real values in the CLP queries.
  • zPCI is now also able to honour vfio DMA limits, if passed via the vfio-pci device, and can trigger the guest to flush its DMA mappings when needed.
  • The s390-ccw bios now tries harder to find a bootable device, if the first device is not suitable. This brings s390x booting a bit closer to what other architectures do.
  • And the usual fixes and cleanups.

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