Protected Virtualization on s390x

The memory and most of the registers of Protected Virtual Machines (PVMs) are encrypted or inaccessible to the hypervisor, effectively prohibiting VM introspection when the VM is running. At rest, PVMs are encrypted and can only be decrypted by the firmware, represented by an entity called Ultravisor, of specific IBM Z machines.

Prerequisites

To run PVMs, a machine with the Protected Virtualization feature, as indicated by the Ultravisor Call facility (stfle bit 158), is required. The Ultravisor needs to be initialized at boot by setting prot_virt=1 on the host’s kernel command line.

Running PVMs requires using the KVM hypervisor.

If those requirements are met, the capability KVM_CAP_S390_PROTECTED will indicate that KVM can support PVMs on that LPAR.

Running a Protected Virtual Machine

To run a PVM you will need to select a CPU model which includes the Unpack facility (stfle bit 161 represented by the feature unpack/S390_FEAT_UNPACK), and add these options to the command line:

-object s390-pv-guest,id=pv0 \
-machine confidential-guest-support=pv0

Adding these options will:

  • Ensure the unpack facility is available

  • Enable the IOMMU by default for all I/O devices

  • Initialize the PV mechanism

Passthrough (vfio) devices are currently not supported.

Host huge page backings are not supported. However guests can use huge pages as indicated by its facilities.

Boot Process

A secure guest image can either be loaded from disk or supplied on the QEMU command line. Booting from disk is done by the unmodified s390-ccw BIOS. I.e., the bootmap is interpreted, multiple components are read into memory and control is transferred to one of the components (zipl stage3). Stage3 does some fixups and then transfers control to some program residing in guest memory, which is normally the OS kernel. The secure image has another component prepended (stage3a) that uses the new diag308 subcodes 8 and 10 to trigger the transition into secure mode.

Booting from the image supplied on the QEMU command line requires that the file passed via -kernel has the same memory layout as would result from the disk boot. This memory layout includes the encrypted components (kernel, initrd, cmdline), the stage3a loader and metadata. In case this boot method is used, the command line options -initrd and -cmdline are ineffective. The preparation of a PVM image is done via the genprotimg tool from the s390-tools collection.