[PATCH v8 14/15] docs: Add protvirt docs

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

Lets add some documentation for the Protected VM functionality.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
---
 docs/system/index.rst    |  1 +
 docs/system/protvirt.rst | 56 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+)
 create mode 100644 docs/system/protvirt.rst

diff --git a/docs/system/index.rst b/docs/system/index.rst
index 6e5f20fa13..74afbd7cc3 100644
--- a/docs/system/index.rst
+++ b/docs/system/index.rst
@@ -34,3 +34,4 @@ Contents:
    deprecated
    build-platforms
    license
+   protvirt
diff --git a/docs/system/protvirt.rst b/docs/system/protvirt.rst
new file mode 100644
index 0000000000..6c8cf0f791
--- /dev/null
+++ b/docs/system/protvirt.rst
@@ -0,0 +1,56 @@
+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
+which is 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 kernel command line.
+
+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
+`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). Stag3 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 via 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 by genprotimg of the s390-tools package.
-- 
2.20.1