Re: [PATCH v4 1/2] xen: EXPERT clean-up and introduce UNSUPPORTED

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.01.2021 12:17, Bertrand Marquis wrote:
> 
> 
>> On 26 Jan 2021, at 11:11, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 26.01.2021 12:06, Bertrand Marquis wrote:
>>>> On 26 Jan 2021, at 09:22, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>> On 25.01.2021 22:27, Stefano Stabellini wrote:
>>>>> @@ -77,7 +77,7 @@ config SBSA_VUART_CONSOLE
>>>>>     SBSA Generic UART implements a subset of ARM PL011 UART.
>>>>>
>>>>> config ARM_SSBD
>>>>> - bool "Speculative Store Bypass Disable" if EXPERT
>>>>> + bool "Speculative Store Bypass Disable (UNSUPPORTED)" if UNSUPPORTED
>>>>>   depends on HAS_ALTERNATIVE
>>>>>   default y
>>>>>   help
>>>>> @@ -87,7 +87,7 @@ config ARM_SSBD
>>>>>     If unsure, say Y.
>>>>>
>>>>> config HARDEN_BRANCH_PREDICTOR
>>>>> - bool "Harden the branch predictor against aliasing attacks" if EXPERT
>>>>> + bool "Harden the branch predictor against aliasing attacks 
>>>>> (UNSUPPORTED)" if UNSUPPORTED
>>>>>   default y
>>>>>   help
>>>>>     Speculation attacks against some high-performance processors rely on
>>>>
>>>> Both of these default to y and have their _prompt_
>>>> conditional upon EXPERT. Which means only an expert can turn them
>>>> _off_. Which doesn't make it look like these are unsupported? Or
>>>> if anything, turning them off is unsupported?
>>>
>>> ...You could see that as EXPERT/UNSUPPORTED options can only be
>>> “modified” from their default value if EXPERT/UNSUPPORTED is activated.
>>
>> But this is nothing you can recognize when configuring Xen
>> (i.e. seeing just these prompts).
> 
> Maybe something we could explain more clearly in the UNSUPPORTED/EXPERT
> config parameters instead ?
> We could also make that more clear in the help of such parameters directly.
> 
> I do not see how we could make that more clear directly in the prompt (as
> making it too long is not a good solution).

My main request is that such tags be added only if there's
absolutely no ambiguity. Anything else (requiring longer
explanations in many cases) should be expressed in the help
text of the option, or in yet other ways (a referral to
SUPPORT.md comes to mind).

Jan