ppc: Officially deprecate the CPU "compat" property

Server class POWER CPUs have a "compat" property, which was obsoleted
by commit 7843c0d60d and replaced by a "max-cpu-compat" property on the
pseries machine type. A hack was introduced so that passing "compat" to
-cpu would still produce the desired effect, for the sake of backward
compatibility : it strips the "compat" option from the CPU properties
and applies internally it to the pseries machine. The accessors of the
"compat" property were updated to do nothing but warn the user about the
deprecated status when doing something like:

$ qemu-system-ppc64 -global POWER9-family-powerpc64-cpu.compat=power9
qemu-system-ppc64: warning: CPU 'compat' property is deprecated and has no
 effect; use max-cpu-compat machine property instead

This was merged during the QEMU 2.10 timeframe, a few weeks before we
formalized our deprecation process. As a consequence, the "compat"
property fell through the cracks and was never listed in the officialy
deprecated features.

We are now eight QEMU versions later, it is largely time to mention it
in qemu-deprecated.texi. Also, since -global XXX-powerpc64-cpu.compat=
has been emitting warnings since QEMU 2.10 and the usual way of setting
CPU properties is with -cpu, completely remove the "compat" property.
Keep the hack so that -cpu XXX,compat= stays functional some more time,
as required by our deprecation process.

The now empty powerpc_servercpu_properties[] list which was introduced
for "compat" and never had any other use is removed on the way. We can
re-add it in the future if the need for a server class POWER CPU specific
property arises again.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <158274357799.140275.12263135811731647490.stgit@bahia.lan>
[dwg: Convert from .texi to .rst to match upstream change]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

docs/system/deprecated.rst

@@ -289,6 +289,13 @@ The RISC-V no MMU cpus have been depcreated. The two CPUs: ``rv32imacu-nommu`` a
 ``rv64imacu-nommu`` should no longer be used. Instead the MMU status can be specified
 via the CPU ``mmu`` option when using the ``rv32`` or ``rv64`` CPUs.
 
+``compat`` property of server class POWER CPUs (since 5.0)
+''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+The ``compat`` property used to set backwards compatibility modes for
+the processor has been deprecated. The ``max-cpu-compat`` property of
+the ``pseries`` machine type should be used instead.
+
 System emulator devices
 -----------------------