Adding upstream version 6.1.76.upstream/6.1.76upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. 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>`_