protvirt.rst (2608B)
1 Protected Virtualization on s390x 2 ================================= 3 4 The memory and most of the registers of Protected Virtual Machines 5 (PVMs) are encrypted or inaccessible to the hypervisor, effectively 6 prohibiting VM introspection when the VM is running. At rest, PVMs are 7 encrypted and can only be decrypted by the firmware, represented by an 8 entity called Ultravisor, of specific IBM Z machines. 9 10 11 Prerequisites 12 ------------- 13 14 To run PVMs, a machine with the Protected Virtualization feature, as 15 indicated by the Ultravisor Call facility (stfle bit 158), is 16 required. The Ultravisor needs to be initialized at boot by setting 17 ``prot_virt=1`` on the host's kernel command line. 18 19 Running PVMs requires using the KVM hypervisor. 20 21 If those requirements are met, the capability ``KVM_CAP_S390_PROTECTED`` 22 will indicate that KVM can support PVMs on that LPAR. 23 24 25 Running a Protected Virtual Machine 26 ----------------------------------- 27 28 To run a PVM you will need to select a CPU model which includes the 29 ``Unpack facility`` (stfle bit 161 represented by the feature 30 ``unpack``/``S390_FEAT_UNPACK``), and add these options to the command line:: 31 32 -object s390-pv-guest,id=pv0 \ 33 -machine confidential-guest-support=pv0 34 35 Adding these options will: 36 37 * Ensure the ``unpack`` facility is available 38 * Enable the IOMMU by default for all I/O devices 39 * Initialize the PV mechanism 40 41 Passthrough (vfio) devices are currently not supported. 42 43 Host huge page backings are not supported. However guests can use huge 44 pages as indicated by its facilities. 45 46 47 Boot Process 48 ------------ 49 50 A secure guest image can either be loaded from disk or supplied on the 51 QEMU command line. Booting from disk is done by the unmodified 52 s390-ccw BIOS. I.e., the bootmap is interpreted, multiple components 53 are read into memory and control is transferred to one of the 54 components (zipl stage3). Stage3 does some fixups and then transfers 55 control to some program residing in guest memory, which is normally 56 the OS kernel. The secure image has another component prepended 57 (stage3a) that uses the new diag308 subcodes 8 and 10 to trigger the 58 transition into secure mode. 59 60 Booting from the image supplied on the QEMU command line requires that 61 the file passed via -kernel has the same memory layout as would result 62 from the disk boot. This memory layout includes the encrypted 63 components (kernel, initrd, cmdline), the stage3a loader and 64 metadata. In case this boot method is used, the command line 65 options -initrd and -cmdline are ineffective. The preparation of a PVM 66 image is done via the ``genprotimg`` tool from the s390-tools 67 collection.