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[PATCH] docs/system: Deprecate `-cpu ...,+feature,-feature` syntax

From: Eduardo Habkost
Subject: [PATCH] docs/system: Deprecate `-cpu ...,+feature,-feature` syntax
Date: Wed, 20 Jan 2021 15:12:41 -0500

The ordering semantics of +feature/-feature is tricky and not
obvious, and it requires a custom option parser.  Deprecate that
syntax so we can eventually remove the custom -cpu option parser
and plus_features/minus_features global variables in i386.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
 docs/system/deprecated.rst | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
index e20bfcb17a4..2c4b8d4b78b 100644
--- a/docs/system/deprecated.rst
+++ b/docs/system/deprecated.rst
@@ -127,6 +127,20 @@ Drives with interface types other than ``if=none`` are for 
 devices.  It is possible to use drives the board doesn't pick up with
 -device.  This usage is now deprecated.  Use ``if=none`` instead.
+``-cpu`` ``+feature`` and ``-feature`` syntax (since 6.0.0)
+The ``-cpu ...,+feature`` and ``-cpu ...,-feature`` syntax for
+enabling and disabling CPU features is deprecated.  The ``-cpu
+...,feature=on`` or ``-cpu ...,feature=off`` should be used
+Note that the ordering semantics of ``-cpu ...,-feature,+feature``
+is different from ``-cpu ...,feature=off,feature=on``.  With the
+former, the feature got disabled because ``-feature`` had
+precedence, but with the latter the feature will be enabled
+because options are applied in the order they appear.
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