[PATCH v6 17/18] 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 | 57 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+)
 create mode 100644 docs/system/protvirt.rst

diff --git a/docs/system/index.rst b/docs/system/index.rst
index 1a4b2c82ac..d2dc63b973 100644
--- a/docs/system/index.rst
+++ b/docs/system/index.rst
@@ -16,3 +16,4 @@ Contents:
 
    qemu-block-drivers
    vfio-ap
+   protvirt
diff --git a/docs/system/protvirt.rst b/docs/system/protvirt.rst
new file mode 100644
index 0000000000..a1902cc47c
--- /dev/null
+++ b/docs/system/protvirt.rst
@@ -0,0 +1,57 @@
+Protected Virtualization on s390x
+=================================
+
+The memory and most of the register contents of Protected Virtual
+Machines (PVMs) are 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 of specific IBM Z
+machines.
+
+
+Prerequisites
+-------------
+
+To run PVMs, you need to have a machine with the Protected
+Virtualization feature, which is indicated by the Ultravisor Call
+facility (stfle bit 158). This is a KVM only feature, therefore you
+need a KVM which is able to support PVMs and activate the Ultravisor
+initialization 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 move into protected mode, the
+`Unpack facility` (stfle bit 161) 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. The guest however can use
+huge pages as indicated by its facilities.
+
+
+Boot Process
+------------
+
+A secure guest image can be both booted from disk and using the QEMU
+command line. Booting from disk is done by the unmodified s390-ccw
+BIOS. I.e., the bootmap is interpreted and a number of components is
+read into memory and control is transferred to one of the components
+(zipl stage3), which 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)
+which uses the new diag308 subcodes 8 and 10 to trigger the transition
+into secure mode.
+
+Booting from the 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 secure guest image is done by a
+program (name tbd) of the s390-tools package.
-- 
2.20.1