Documentation: create/move s390x documentation

Create a subdirectory for s390x under docs/system/ and move the
existing vfio-ap documentation there.

Create an initial document describing s390x system emulation.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20200318103940.1169-1-cohuck@redhat.com>

MAINTAINERS

@@ -101,7 +101,8 @@ F: include/hw/watchdog/wdt_diag288.h
 F: pc-bios/s390-ccw/
 F: pc-bios/s390-ccw.img
 F: target/s390x/
-F: docs/vfio-ap.txt
+F: docs/system/target-s390x.rst
+F: docs/system/s390x/
 F: tests/migration/s390x/
 K: ^Subject:.*(?i)s390x?
 T: git https://github.com/cohuck/qemu.git s390-next
@@ -1611,7 +1612,7 @@ F: hw/s390x/ap-bridge.c
 F: include/hw/s390x/ap-device.h
 F: include/hw/s390x/ap-bridge.h
 F: hw/vfio/ap.c
-F: docs/system/vfio-ap.rst
+F: docs/system/s390x/vfio-ap.rst
 L: qemu-s390x@nongnu.org
 
 vhost

docs/system/index.rst

@@ -30,7 +30,6 @@ Contents:
    managed-startup
    targets
    security
-   vfio-ap
    deprecated
    build-platforms
    license

docs/system/target-s390x.rst

@@ -0,0 +1,26 @@
+.. _s390x-System-emulator:
+
+s390x System emulator
+---------------------
+
+QEMU can emulate z/Architecture (in particular, 64 bit) s390x systems
+via the ``qemu-system-s390x`` binary. Only one machine type,
+``s390-ccw-virtio``, is supported (with versioning for compatibility
+handling).
+
+When using KVM as accelerator, QEMU can emulate CPUs up to the generation
+of the host. When using the default cpu model with TCG as accelerator,
+QEMU will emulate a subset of z13 cpu features that should be enough to run
+distributions built for the z13.
+
+Device support
+==============
+
+QEMU will not emulate most of the traditional devices found under LPAR or
+z/VM; virtio devices (especially using virtio-ccw) make up the bulk of
+the available devices. Passthrough of host devices via vfio-pci, vfio-ccw,
+or vfio-ap is also available.
+
+.. toctree::
+   s390x/vfio-ap
+

docs/system/targets.rst

@@ -17,3 +17,4 @@ Contents:
    target-arm
    target-m68k
    target-xtensa
+   target-s390x