docs: Add a note about mixing bootindex with "-boot order"

Occasionally the users try to mix the bootindex properties with the
"-boot order" parameter - and this likely does not give the expected
results. So let's add a proper statement that these two concepts
should not be used together.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1488303601-23741-1-git-send-email-thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/bootindex.txt

@@ -41,3 +41,12 @@ has three bootable devices target1, target3, target5 connected to it,
 the option ROM will have a boot method for each of them, but it is not
 possible to map from boot method back to a specific target.  This is a
 shortcoming of the PC BIOS boot specification.
+
+== Mixing bootindex and boot order parameters ==
+
+Note that it does not make sense to use the bootindex property together
+with the "-boot order=..." (or "-boot once=...") parameter. The guest
+firmware implementations normally either support the one or the other,
+but not both parameters at the same time. Mixing them will result in
+undefined behavior, and thus the guest firmware will likely not boot
+from the expected devices.

qemu-options.hx

@@ -252,7 +252,10 @@ drive letters depend on the target architecture. The x86 PC uses: a, b
 (floppy 1 and 2), c (first hard disk), d (first CD-ROM), n-p (Etherboot
 from network adapter 1-4), hard disk boot is the default. To apply a
 particular boot order only on the first startup, specify it via
-@option{once}.
+@option{once}. Note that the @option{order} or @option{once} parameter
+should not be used together with the @option{bootindex} property of
+devices, since the firmware implementations normally do not support both
+at the same time.
 
 Interactive boot menus/prompts can be enabled via @option{menu=on} as far
 as firmware/BIOS supports them. The default is non-interactive boot.