1Protected Virtualization on s390x 2================================= 3 4The memory and most of the registers of Protected Virtual Machines 5(PVMs) are encrypted or inaccessible to the hypervisor, effectively 6prohibiting VM introspection when the VM is running. At rest, PVMs are 7encrypted and can only be decrypted by the firmware, represented by an 8entity called Ultravisor, of specific IBM Z machines. 9 10 11Prerequisites 12------------- 13 14To run PVMs, a machine with the Protected Virtualization feature, as 15indicated by the Ultravisor Call facility (stfle bit 158), is 16required. The Ultravisor needs to be initialized at boot by setting 17`prot_virt=1` on the host's kernel command line. 18 19Running PVMs requires using the KVM hypervisor. 20 21If those requirements are met, the capability `KVM_CAP_S390_PROTECTED` 22will indicate that KVM can support PVMs on that LPAR. 23 24 25Running a Protected Virtual Machine 26----------------------------------- 27 28To 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 35Adding 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 41Passthrough (vfio) devices are currently not supported. 42 43Host huge page backings are not supported. However guests can use huge 44pages as indicated by its facilities. 45 46 47Boot Process 48------------ 49 50A secure guest image can either be loaded from disk or supplied on the 51QEMU command line. Booting from disk is done by the unmodified 52s390-ccw BIOS. I.e., the bootmap is interpreted, multiple components 53are read into memory and control is transferred to one of the 54components (zipl stage3). Stage3 does some fixups and then transfers 55control to some program residing in guest memory, which is normally 56the OS kernel. The secure image has another component prepended 57(stage3a) that uses the new diag308 subcodes 8 and 10 to trigger the 58transition into secure mode. 59 60Booting from the image supplied on the QEMU command line requires that 61the file passed via -kernel has the same memory layout as would result 62from the disk boot. This memory layout includes the encrypted 63components (kernel, initrd, cmdline), the stage3a loader and 64metadata. In case this boot method is used, the command line 65options -initrd and -cmdline are ineffective. The preparation of a PVM 66image is done via the `genprotimg` tool from the s390-tools 67collection. 68