.. SPDX-License-Identifier: GPL-2.0

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s390 (IBM Z) Ultravisor and Protected VMs
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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
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>`_