[PATCH v5 18/19] docs/system: Update documentation for s390x IPL

[jrossi@linux.ibm.com]

From: Jared Rossi <jrossi@linux.ibm.com>

Update docs to show that s390x PC BIOS can support more than one boot device.

Signed-off-by: Jared Rossi <jrossi@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
---
 docs/system/bootindex.rst         | 7 ++++---
 docs/system/s390x/bootdevices.rst | 9 ++++++---
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/docs/system/bootindex.rst b/docs/system/bootindex.rst
index 8b057f812f..988f7b3beb 100644
--- a/docs/system/bootindex.rst
+++ b/docs/system/bootindex.rst
@@ -49,10 +49,11 @@ Limitations
 -----------
 
 Some firmware has limitations on which devices can be considered for
-booting.  For instance, the PC BIOS boot specification allows only one
-disk to be bootable.  If boot from disk fails for some reason, the BIOS
+booting.  For instance, the x86 PC BIOS boot specification allows only one
+disk to be bootable.  If boot from disk fails for some reason, the x86 BIOS
 won't retry booting from other disk.  It can still try to boot from
-floppy or net, though.
+floppy or net, though. In the case of s390x BIOS, the BIOS will try up to
+8 total devices, any number of which may be disks.
 
 Sometimes, firmware cannot map the device path QEMU wants firmware to
 boot from to a boot method.  It doesn't happen for devices the firmware
diff --git a/docs/system/s390x/bootdevices.rst b/docs/system/s390x/bootdevices.rst
index c97efb8fc0..1a1a764c1c 100644
--- a/docs/system/s390x/bootdevices.rst
+++ b/docs/system/s390x/bootdevices.rst
@@ -6,9 +6,7 @@ Booting with bootindex parameter
 
 For classical mainframe guests (i.e. LPAR or z/VM installations), you always
 have to explicitly specify the disk where you want to boot from (or "IPL" from,
-in s390x-speak -- IPL means "Initial Program Load"). In particular, there can
-also be only one boot device according to the architecture specification, thus
-specifying multiple boot devices is not possible (yet).
+in s390x-speak -- IPL means "Initial Program Load").
 
 So for booting an s390x guest in QEMU, you should always mark the
 device where you want to boot from with the ``bootindex`` property, for
@@ -17,6 +15,11 @@ example::
  qemu-system-s390x -drive if=none,id=dr1,file=guest.qcow2 \
                    -device virtio-blk,drive=dr1,bootindex=1
 
+Multiple devices may have a bootindex. The lowest bootindex is assigned to the
+device to IPL first.  If the IPL fails for the first, the device with the second
+lowest bootindex will be tried and so on until IPL is successful or there are no
+remaining boot devices to try.
+
 For booting from a CD-ROM ISO image (which needs to include El-Torito boot
 information in order to be bootable), it is recommended to specify a ``scsi-cd``
 device, for example like this::
-- 
2.45.1