1.. SPDX-License-Identifier: GPL-2.0 2 3========================================= 4s390 (IBM Z) Ultravisor and Protected VMs 5========================================= 6 7Summary 8------- 9Protected virtual machines (PVM) are KVM VMs that do not allow KVM to 10access VM state like guest memory or guest registers. Instead, the 11PVMs are mostly managed by a new entity called Ultravisor (UV). The UV 12provides an API that can be used by PVMs and KVM to request management 13actions. 14 15Each guest starts in non-protected mode and then may make a request to 16transition into protected mode. On transition, KVM registers the guest 17and its VCPUs with the Ultravisor and prepares everything for running 18it. 19 20The Ultravisor will secure and decrypt the guest's boot memory 21(i.e. kernel/initrd). It will safeguard state changes like VCPU 22starts/stops and injected interrupts while the guest is running. 23 24As access to the guest's state, such as the SIE state description, is 25normally needed to be able to run a VM, some changes have been made in 26the behavior of the SIE instruction. A new format 4 state description 27has been introduced, where some fields have different meanings for a 28PVM. SIE exits are minimized as much as possible to improve speed and 29reduce exposed guest state. 30 31 32Interrupt injection 33------------------- 34Interrupt injection is safeguarded by the Ultravisor. As KVM doesn't 35have access to the VCPUs' lowcores, injection is handled via the 36format 4 state description. 37 38Machine check, external, IO and restart interruptions each can be 39injected on SIE entry via a bit in the interrupt injection control 40field (offset 0x54). If the guest cpu is not enabled for the interrupt 41at the time of injection, a validity interception is recognized. The 42format 4 state description contains fields in the interception data 43block where data associated with the interrupt can be transported. 44 45Program and Service Call exceptions have another layer of 46safeguarding; they can only be injected for instructions that have 47been intercepted into KVM. The exceptions need to be a valid outcome 48of an instruction emulation by KVM, e.g. we can never inject a 49addressing exception as they are reported by SIE since KVM has no 50access to the guest memory. 51 52 53Mask notification interceptions 54------------------------------- 55KVM cannot intercept lctl(g) and lpsw(e) anymore in order to be 56notified when a PVM enables a certain class of interrupt. As a 57replacement, two new interception codes have been introduced: One 58indicating that the contents of CRs 0, 6, or 14 have been changed, 59indicating different interruption subclasses; and one indicating that 60PSW bit 13 has been changed, indicating that a machine check 61intervention was requested and those are now enabled. 62 63Instruction emulation 64--------------------- 65With the format 4 state description for PVMs, the SIE instruction already 66interprets more instructions than it does with format 2. It is not able 67to interpret every instruction, but needs to hand some tasks to KVM; 68therefore, the SIE and the ultravisor safeguard emulation inputs and outputs. 69 70The control structures associated with SIE provide the Secure 71Instruction Data Area (SIDA), the Interception Parameters (IP) and the 72Secure Interception General Register Save Area. Guest GRs and most of 73the instruction data, such as I/O data structures, are filtered. 74Instruction data is copied to and from the SIDA when needed. Guest 75GRs are put into / retrieved from the Secure Interception General 76Register Save Area. 77 78Only GR values needed to emulate an instruction will be copied into this 79save area and the real register numbers will be hidden. 80 81The Interception Parameters state description field still contains 82the bytes of the instruction text, but with pre-set register values 83instead of the actual ones. I.e. each instruction always uses the same 84instruction text, in order not to leak guest instruction text. 85This also implies that the register content that a guest had in r<n> 86may be in r<m> from the hypervisor's point of view. 87 88The Secure Instruction Data Area contains instruction storage 89data. Instruction data, i.e. data being referenced by an instruction 90like the SCCB for sclp, is moved via the SIDA. When an instruction is 91intercepted, the SIE will only allow data and program interrupts for 92this instruction to be moved to the guest via the two data areas 93discussed before. Other data is either ignored or results in validity 94interceptions. 95 96 97Instruction emulation interceptions 98----------------------------------- 99There are two types of SIE secure instruction intercepts: the normal 100and the notification type. Normal secure instruction intercepts will 101make the guest pending for instruction completion of the intercepted 102instruction type, i.e. on SIE entry it is attempted to complete 103emulation of the instruction with the data provided by KVM. That might 104be a program exception or instruction completion. 105 106The notification type intercepts inform KVM about guest environment 107changes due to guest instruction interpretation. Such an interception 108is recognized, for example, for the store prefix instruction to provide 109the new lowcore location. On SIE reentry, any KVM data in the data areas 110is ignored and execution continues as if the guest instruction had 111completed. For that reason KVM is not allowed to inject a program 112interrupt. 113 114Links 115----- 116`KVM Forum 2019 presentation <https://static.sched.com/hosted_files/kvmforum2019/3b/ibm_protected_vms_s390x.pdf>`_ 117