[PATCH v10 15/16] docs: system: Add protvirt docs

[Janosch Frank <frankja@linux.ibm.com>]

Let's add some documentation for the Protected VM functionality.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 docs/system/s390x/protvirt.rst | 59 ++++++++++++++++++++++++++++++++++
 docs/system/target-s390x.rst   |  5 +++
 2 files changed, 64 insertions(+)
 create mode 100644 docs/system/s390x/protvirt.rst

diff --git a/docs/system/s390x/protvirt.rst b/docs/system/s390x/protvirt.rst
new file mode 100644
index 0000000000000000..71172511632f7bcb
--- /dev/null
+++ b/docs/system/s390x/protvirt.rst
@@ -0,0 +1,59 @@
+Protected Virtualization on s390x
+=================================
+
+The memory and most of the registers of Protected Virtual Machines
+(PVMs) are encrypted or inaccessible to the hypervisor, effectively
+prohibiting VM introspection when the VM is running. At rest, PVMs are
+encrypted and can only be decrypted by the firmware, represented by an
+entity called Ultravisor, of specific IBM Z machines.
+
+
+Prerequisites
+-------------
+
+To run PVMs, a machine with the Protected Virtualization feature, as
+indicated by the Ultravisor Call facility (stfle bit 158), is
+required. The Ultravisor needs to be initialized at boot by setting
+`prot_virt=1` on the host's kernel command line.
+
+Running PVMs requires using the KVM hypervisor.
+
+If those requirements are met, the capability `KVM_CAP_S390_PROTECTED`
+will indicate that KVM can support PVMs on that LPAR.
+
+
+QEMU Settings
+-------------
+
+To indicate to the VM that it can transition into protected mode, the
+`Unpack facility` (stfle bit 161 represented by the feature
+`unpack`/`S390_FEAT_UNPACK`) needs to be part of the cpu model of
+the VM.
+
+All I/O devices need to use the IOMMU.
+Passthrough (vfio) devices are currently not supported.
+
+Host huge page backings are not supported. However guests can use huge
+pages as indicated by its facilities.
+
+
+Boot Process
+------------
+
+A secure guest image can either be loaded from disk or supplied on the
+QEMU command line. Booting from disk is done by the unmodified
+s390-ccw BIOS. I.e., the bootmap is interpreted, multiple components
+are read into memory and control is transferred to one of the
+components (zipl stage3). Stage3 does some fixups and then transfers
+control to some program residing in guest memory, which is normally
+the OS kernel. The secure image has another component prepended
+(stage3a) that uses the new diag308 subcodes 8 and 10 to trigger the
+transition into secure mode.
+
+Booting from the image supplied on the QEMU command line requires that
+the file passed via -kernel has the same memory layout as would result
+from the disk boot. This memory layout includes the encrypted
+components (kernel, initrd, cmdline), the stage3a loader and
+metadata. In case this boot method is used, the command line
+options -initrd and -cmdline are ineffective. The preparation of a PVM
+image is done via the `genprotimg` tool from the s390-tools package.
diff --git a/docs/system/target-s390x.rst b/docs/system/target-s390x.rst
index 4c8b7cdd66154d26..8d97158dc804af1c 100644
--- a/docs/system/target-s390x.rst
+++ b/docs/system/target-s390x.rst
@@ -24,3 +24,8 @@ or vfio-ap is also available.
 .. toctree::
    s390x/vfio-ap
 
+Architectural features
+~~~~~~~~
+
+.. toctree::
+   s390x/protvirt
-- 
2.25.1