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diff --git a/Documentation/virt/kvm/s390-pv.rst b/Documentation/virt/kvm/s390-pv.rst new file mode 100644 index 000000000..8e41a3b63 --- /dev/null +++ b/Documentation/virt/kvm/s390-pv.rst @@ -0,0 +1,116 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================================= +s390 (IBM Z) Ultravisor and Protected VMs +========================================= + +Summary +------- +Protected virtual machines (PVM) are KVM VMs that do not allow KVM to +access VM state like guest memory or guest registers. Instead, the +PVMs are mostly managed by a new entity called Ultravisor (UV). The UV +provides an API that can be used by PVMs and KVM to request management +actions. + +Each guest starts in non-protected mode and then may make a request to +transition into protected mode. On transition, KVM registers the guest +and its VCPUs with the Ultravisor and prepares everything for running +it. + +The Ultravisor will secure and decrypt the guest's boot memory +(i.e. kernel/initrd). It will safeguard state changes like VCPU +starts/stops and injected interrupts while the guest is running. + +As access to the guest's state, such as the SIE state description, is +normally needed to be able to run a VM, some changes have been made in +the behavior of the SIE instruction. A new format 4 state description +has been introduced, where some fields have different meanings for a +PVM. SIE exits are minimized as much as possible to improve speed and +reduce exposed guest state. + + +Interrupt injection +------------------- +Interrupt injection is safeguarded by the Ultravisor. As KVM doesn't +have access to the VCPUs' lowcores, injection is handled via the +format 4 state description. + +Machine check, external, IO and restart interruptions each can be +injected on SIE entry via a bit in the interrupt injection control +field (offset 0x54). If the guest cpu is not enabled for the interrupt +at the time of injection, a validity interception is recognized. The +format 4 state description contains fields in the interception data +block where data associated with the interrupt can be transported. + +Program and Service Call exceptions have another layer of +safeguarding; they can only be injected for instructions that have +been intercepted into KVM. The exceptions need to be a valid outcome +of an instruction emulation by KVM, e.g. we can never inject a +addressing exception as they are reported by SIE since KVM has no +access to the guest memory. + + +Mask notification interceptions +------------------------------- +KVM cannot intercept lctl(g) and lpsw(e) anymore in order to be +notified when a PVM enables a certain class of interrupt. As a +replacement, two new interception codes have been introduced: One +indicating that the contents of CRs 0, 6, or 14 have been changed, +indicating different interruption subclasses; and one indicating that +PSW bit 13 has been changed, indicating that a machine check +intervention was requested and those are now enabled. + +Instruction emulation +--------------------- +With the format 4 state description for PVMs, the SIE instruction already +interprets more instructions than it does with format 2. It is not able +to interpret every instruction, but needs to hand some tasks to KVM; +therefore, the SIE and the ultravisor safeguard emulation inputs and outputs. + +The control structures associated with SIE provide the Secure +Instruction Data Area (SIDA), the Interception Parameters (IP) and the +Secure Interception General Register Save Area. Guest GRs and most of +the instruction data, such as I/O data structures, are filtered. +Instruction data is copied to and from the SIDA when needed. Guest +GRs are put into / retrieved from the Secure Interception General +Register Save Area. + +Only GR values needed to emulate an instruction will be copied into this +save area and the real register numbers will be hidden. + +The Interception Parameters state description field still contains +the bytes of the instruction text, but with pre-set register values +instead of the actual ones. I.e. each instruction always uses the same +instruction text, in order not to leak guest instruction text. +This also implies that the register content that a guest had in r<n> +may be in r<m> from the hypervisor's point of view. + +The Secure Instruction Data Area contains instruction storage +data. Instruction data, i.e. data being referenced by an instruction +like the SCCB for sclp, is moved via the SIDA. When an instruction is +intercepted, the SIE will only allow data and program interrupts for +this instruction to be moved to the guest via the two data areas +discussed before. Other data is either ignored or results in validity +interceptions. + + +Instruction emulation interceptions +----------------------------------- +There are two types of SIE secure instruction intercepts: the normal +and the notification type. Normal secure instruction intercepts will +make the guest pending for instruction completion of the intercepted +instruction type, i.e. on SIE entry it is attempted to complete +emulation of the instruction with the data provided by KVM. That might +be a program exception or instruction completion. + +The notification type intercepts inform KVM about guest environment +changes due to guest instruction interpretation. Such an interception +is recognized, for example, for the store prefix instruction to provide +the new lowcore location. On SIE reentry, any KVM data in the data areas +is ignored and execution continues as if the guest instruction had +completed. For that reason KVM is not allowed to inject a program +interrupt. + +Links +----- +`KVM Forum 2019 presentation <https://static.sched.com/hosted_files/kvmforum2019/3b/ibm_protected_vms_s390x.pdf>`_ |