| 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. |
