Re: [RFC PATCH] xen: EXPERT clean-up

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.10.2020 01:24, Stefano Stabellini wrote:
> --- a/xen/Kconfig
> +++ b/xen/Kconfig
> @@ -35,14 +35,13 @@ config DEFCONFIG_LIST
>       default ARCH_DEFCONFIG
>  
>  config EXPERT
> -     bool "Configure standard Xen features (expert users)"
> +     bool "Configure EXPERT features"
>       help
> -       This option allows certain base Xen options and settings
> -       to be disabled or tweaked. This is for specialized environments
> -       which can tolerate a "non-standard" Xen.
> -       Only use this if you really know what you are doing.
> -       Xen binaries built with this option enabled are not security
> -       supported.
> +       This option allows certain experimental (see SUPPORT.md) Xen
> +       options and settings to be enabled/disabled. This is for
> +       specialized environments which can tolerate a "non-standard" Xen.
> +       Only use this if you really know what you are doing.  Xen binaries
> +       built with this option enabled are not security supported.
>       default n

I'm definitely in favor of this - it was more than once that I
wondered about the prompt text.

> @@ -79,8 +79,8 @@ config SBSA_VUART_CONSOLE
>         SBSA Generic UART implements a subset of ARM PL011 UART.
>  
>  config ARM_SSBD
> -     bool "Speculative Store Bypass Disable" if EXPERT
> -     depends on HAS_ALTERNATIVE
> +     bool "Speculative Store Bypass Disable"
> +     depends on HAS_ALTERNATIVE && EXPERT
>       default y

At the example of this, I'm afraid when the default isn't "n"
(or there's no default directive at all, as ought to be
equivalent to and preferred over "default n"), such a
transformation is not functionally identical: Before your
change, with !EXPERT this option defaults to y. After your
change this option is unavailable (which resolves to it being
off for all consuming purposes).

IOW there are reasons to have "if ..." attached to the prompts
(for this construct indeed only making the prompt conditional,
not the entire option), but there are also cases where the
cleanup you do is indeed desirable / helpful.

Jan