docs: Describe protected virtualization guest setup

Protected virtualization/IBM Secure Execution for Linux protects
guest memory and state from the host.

Add some basic information about technology and a brief guide
on setting up secure guests with libvirt.
Signed-off-by: NViktor Mihajlovski <mihajlov@linux.ibm.com>
Signed-off-by: NBoris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: NPaulo de Rezende Pinatti <ppinatti@linux.ibm.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

docs/kbase.html.in

@@ -14,6 +14,9 @@
         <dt><a href="kbase/secureusage.html">Secure usage</a></dt>
         <dd>Secure usage of the libvirt APIs</dd>
 
+        <dt><a href="kbase/s390_protected_virt.html">Protected virtualization on s390</a></dt>
+        <dd>Running secure s390 guests with IBM Secure Execution</dd>
+
         <dt><a href="kbase/launch_security_sev.html">Launch security</a></dt>
         <dd>Securely launching VMs with AMD SEV</dd>
 

docs/kbase/s390_protected_virt.rst

+================================
+Protected Virtualization on s390
+================================
+
+.. contents::
+
+Overview
+========
+
+Protected virtualization, also known as IBM Secure Execution is a
+hardware-based privacy protection technology for s390x (IBM Z).
+It allows to execute virtual machines such that the host system
+has no access to a VM's state and memory contents.
+
+Unlike other similar technologies, the memory of a running guest
+is not encrypted but protected by hardware access controls, which
+may only be manipulated by trusted system firmware, called
+ultravisor.
+
+For the cases where the host needs access to guest memory (e.g. for
+paging), it can request pages to be exported to it. The exported page
+will be encrypted with a unique key for the running guest by the
+ultravisor. The ultravisor also computes an integrity value for
+the page, and stores it in a special table, together with the page
+index and a counter. This way it can verify the integrity of
+the page content upon re-import into the guest.
+
+In other cases it may be necessary for a guest to grant the host access
+to dedicated memory regions (e.g. for I/O). The guest can request
+that the ultravisor removes the memory protection from individual
+pages, so that they can be shared with the host. Likewise, the
+guest can undo the sharing.
+
+A secure guest will initially start in a regular non-protected VM.
+The start-up is controlled by a small bootstrap program loaded
+into memory together with encrypted operating system components and
+a control structure (the PV header).
+The operating system components (e.g. Linux kernel, initial RAM
+file system, kernel parameters) are encrypted and integrity
+protected. The component encryption keys and integrity values are
+stored in the PV header.
+The PV header is wrapped with a public key belonging to a specific
+system (in fact it can be wrapped with multiple such keys). The
+matching private key is only accessible by trusted hardware and
+firmware in that specific system.
+Consequently, such a secure guest boot image can only be run on the
+systems it has been prepared for. Its contents can't be decrypted
+without access to the private key and it can't be modified as
+it is integrity protected.
+
+Host Requirements
+=================
+
+IBM Secure Execution for Linux has some hardware and firmware
+requirements. The system hardware must be an IBM z15 (or newer),
+or an IBM LinuxONE III (or newer).
+
+It is also necessary that the IBM Secure Execution feature is
+enabled for that system. With libvirt >= 6.5.0 you can run
+``libvirt-host--validate`` or otherwise check for facility '158', e.g.
+
+::
+
+   $ grep facilities /proc/cpuinfo | grep 158
+
+The kernel must include the protected virtualization support
+which can be verified by checking for the presence of directory
+``/sys/firmware/uv``. It will only be present when both the
+hardware and the kernel support are available.
+
+Finally, the host operating system must donate some memory to
+the ultravisor needed to store memory security information.
+This is achieved by specifying the following kernel command
+line parameter to the host boot configuration
+
+::
+
+   prot_virt=1
+
+
+Guest Requirements
+==================
+
+Guest Boot
+----------
+
+To start a guest in protected virtualization secure mode, the
+boot image must have been prepared first with the program
+``genprotimg`` using the correct public key for this host.
+``genprotimg`` is part of the package ``s390-tools``, or
+``s390-utils``, depending on the Linux distribution being used.
+It can also be found at
+`<https://github.com/ibm-s390-tools/s390-tools/tree/master/genprotimg>`_
+
+The guests have to be configured to use the host CPU model, which
+must contain the ``unpack`` facility indicating ultravisor guest support.
+
+With the following command it's possible to check whether the host
+CPU model satisfies the requirement
+
+::
+
+   $ virsh domcapabilities | grep unpack
+
+which should return
+
+::
+
+   <feature policy='require' name='unpack'/>
+
+Note that on hosts with libvirt < 6.5.0 if the check fails despite
+the host system actually supporting protected virtualization guests,
+this can be caused by a stale libvirt capabilities cache.
+To recover, run the following commands
+
+::
+
+   $ systemctl stop libvirtd
+   $ rm /var/cache/libvirt/qemu/capabilities/*.xml
+   $ systemctl start libvirtd
+
+
+Guest I/O
+---------
+
+Protected virtualization guests support I/O using virtio devices.
+As the virtio data structures of secure guests are not accessible
+by the host, it is necessary to use shared memory ('bounce buffers').
+
+To enable virtio devices to use shared buffers, it is necessary
+to configure them with platform_iommu enabled. This can done by adding
+``iommu='on'`` to the driver element of a virtio device definition in the
+guest's XML, e.g.
+
+::
+
+   <interface type='network'>
+     <source network='default'/>
+     <model type='virtio'/>
+     <driver name='vhost' iommu='on'/>
+   </interface>
+
+It is mandatory to define all virtio bus devices in this way to
+prevent the host from attempting to access protected memory.
+Ballooning will not work and is fenced by QEMU. It should be
+disabled by specifying
+
+::
+
+   <memballoon model='none'/>
+
+Finally, the guest Linux must be instructed to allocate I/O
+buffers using memory shared between host and guest using SWIOTLB.
+This is done by adding ``swiotlb=nnn`` to the guest's kernel command
+line string, where ``nnn`` stands for the number of statically
+allocated 2K entries. A commonly used value for swiotlb is 262144.
+
+Example guest definition
+========================
+
+Minimal domain XML for a protected virtualization guest, essentially
+it's mostly about the ``iommu`` property
+
+::
+
+   <domain type='kvm'>
+     <name>protected</name>
+     <memory unit='KiB'>2048000</memory>
+     <currentMemory unit='KiB'>2048000</currentMemory>
+     <vcpu>1</vcpu>
+     <os>
+       <type arch='s390x'>hvm</type>
+     </os>
+     <cpu mode='host-model'/>
+     <devices>
+       <disk type='file' device='disk'>
+         <driver name='qemu' type='qcow2' cache='none' io='native' iommu='on'>
+         <source file='/var/lib/libvirt/images/protected.qcow2'/>
+         <target dev='vda' bus='virtio'/>
+       </disk>
+       <interface type='network'>
+         <driver iommu='on'/>
+         <source network='default'/>
+         <model type='virtio'/>
+       </interface>
+       <console type='pty'/>
+       <memballoon model='none'/>
+     </devices>
+   </domain>