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

[Daniel P . Berrangé]

On Wed, Jan 20, 2021 at 03:12:41PM -0500, Eduardo Habkost wrote:
> 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(+)

Ideally we would also print a warning on stderr when this deprecated
style is used.

> 
> 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 onboard
>  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
> +instead.
> +
> +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.
> +

Regards,
Daniel
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