Re: [PATCH v4 27/34] qemu-options: New -compat to set policy for deprecated interfaces

[Eric Blake]

On 3/17/20 6:54 AM, Markus Armbruster wrote:
> Policy is separate for input and output.
>
> Input policy can be "accept" (accept silently), or "reject" (reject
> the request with an error).
>
> Output policy can be "accept" (pass on unchanged), or "hide" (filter
> out the deprecated parts).
>
> Default is "accept".  Policies other than "accept" are implemented
> later in this series.
>
> For now, -compat covers only syntactic aspects of QMP, i.e. stuff
> tagged with feature 'deprecated'.  We may want to extend it to cover
> semantic aspects, CLI, and experimental features.
>
> The option is experimental.

On IRC, we decided that it's probably not worth shoe-horning this (and 
the rest of the series) into 5.0, given the experimental nature.  Still, 
I'll go ahead and review, so we can settle on things early in 5.1.

> Signed-off-by: Markus Armbruster <address@hidden>
> ---

> +++ b/qapi/compat.json
> @@ -0,0 +1,51 @@
> +# -*- Mode: Python -*-
> +
> +##
> +# = Compatibility policy
> +##
> +
> +##
> +# @CompatPolicyInput:
> +#
> +# Policy for handling "funny" input.
> +#
> +# @accept: Accept silently
> +# @reject: Reject with an error
> +#
> +# Since: 5.0

Of course, now that we're slipping this, you'll have to s/5.0/5.1/g over 
the remaining patches.  I won't point it out further.

> +##
> +# @CompatPolicy:
> +#
> +# Policy for handling deprecated management interfaces.
> +#
> +# This is intended for testing users of the management interfaces.
> +#
> +# Limitation: covers only syntactic aspects of QMP, i.e. stuff tagged
> +# with feature 'deprecated'.  We may want to extend it to cover
> +# semantic aspects, CLI, and experimental features.

Hiding/rejecting x- interfaces is probably the easiest of these, but I 
agree that leaving this open-ended to add further coverage (or even 
additional modes) is still reasonable.

> +#
> +# @deprecated-input: how to handle deprecated input (default 'accept')
> +# @deprecated-output: how to handle deprecated output (default 'accept')
> +#
> +# Since: 5.0
> +##
> +{ 'struct': 'CompatPolicy',
> +  'data': { '*deprecated-input': 'CompatPolicyInput',
> +            '*deprecated-output': 'CompatPolicyOutput' } }

For example, adding
'*experimental-input': 'CompatPolicyInput'
would make it easy to hard-code failure on attempt to use x-* commands.


> +++ b/include/qapi/compat-policy.h
> @@ -0,0 +1,20 @@
> +/*
> + * Policy for handling "funny" management interfaces
> + *
> + * Copyright (C) 2019 Red Hat, Inc.

You've had this in-tree for a while. I'll leave it up to you if you want 
to add 2020.

> + *
> + * Authors:
> + *  Markus Armbruster <address@hidden>,
> + *

Ending with a comma is odd.  Is the Authors: snippet even necessary, or 
are we better off relying on git history (which tends to be more 
accurate anyway)?

> + * This work is licensed under the terms of the GNU GPL, version 2 or
> + * later.  See the COPYING file in the top-level directory.
> + */

Are we trying to use SPDX tags in more files?

> +++ b/qemu-options.hx
> @@ -3357,6 +3357,26 @@ DEFHEADING()
>   
>   DEFHEADING(Debug/Expert options:)
>   
> +DEF("compat", HAS_ARG, QEMU_OPTION_compat,
> +    "-compat 
> [deprecated-input=accept|reject][,deprecated-output=accept|hide]\n"
> +    "                Policy for handling deprecated management interfaces\n",
> +    QEMU_ARCH_ALL)
> +SRST
> +``-compat 
> [deprecated-input=@var{input-policy}][,deprecated-output=@var{output-policy}]``
> +    Set policy for handling deprecated management interfaces (experimental):

We'll eventually want to drop (experimental), especially if we get all 
the rest of this into 5.1.

But for now this looks like a good start.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org