[PATCH v2] docs/s390x/pcidevices: document pci devices on s390x

[Sebastian Mitterle <smitterl@redhat.com>]

Add some documentation about the zpci device and how
to use it with pci devices on s390x.

Used source: Cornelia Huck's blog post
https://people.redhat.com/~cohuck/2018/02/19/notes-on-pci-on-s390x.html

Signed-off-by: Sebastian Mitterle <smitterl@redhat.com>
---
v2: move section below 'Device support'
---
 docs/system/s390x/pcidevices.rst | 34 ++++++++++++++++++++++++++++++++
 docs/system/target-s390x.rst     |  1 +
 2 files changed, 35 insertions(+)
 create mode 100644 docs/system/s390x/pcidevices.rst

diff --git a/docs/system/s390x/pcidevices.rst b/docs/system/s390x/pcidevices.rst
new file mode 100644
index 0000000000..fec905d6e6
--- /dev/null
+++ b/docs/system/s390x/pcidevices.rst
@@ -0,0 +1,34 @@
+PCI devices on s390x
+====================
+
+PCI devices on s390x work differently than on other architectures.
+
+To start with, using a PCI device requires the additional ``zpci`` device. For example,
+in order to pass a PCI device ``0000:00:00.0`` through you'd specify::
+
+ qemu-system-s390x ... \
+                   -device zpci,uid=1,fid=0,target=hostdev0,id=zpci1 \
+                   -device vfio-pci,host=0000:00:00.0,id=hostdev0
+
+Here, the zpci device is joined with the PCI device via the ``target`` property.
+
+Note that we don't set bus, slot or function here for the guest as is common in other
+PCI implementations. Topology information is not available on s390x. Instead, ``uid``
+and ``fid`` determine how the device is presented to the guest operating system.
+
+In case of Linux, ``uid`` will be used in the ``domain`` part of the PCI identifier, and
+``fid`` identifies the physical slot, i.e.::
+
+ qemu-system-s390x ... \
+                   -device zpci,uid=7,fid=8,target=hostdev0,id=zpci1 \
+                   ...
+
+will be presented in the guest as::
+
+ # lspci -v
+ 0007:00:00.0 ...
+ Physical Slot: 00000008
+ ...
+
+Finally, note that you might have to enable the ``zpci`` feature in the cpu model in oder to use
+it.
diff --git a/docs/system/target-s390x.rst b/docs/system/target-s390x.rst
index c636f64113..f6f11433c7 100644
--- a/docs/system/target-s390x.rst
+++ b/docs/system/target-s390x.rst
@@ -26,6 +26,7 @@ or vfio-ap is also available.
    s390x/css
    s390x/3270
    s390x/vfio-ccw
+   s390x/pcidevices
 
 Architectural features
 ======================
-- 
2.37.3