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