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

[Jan Beulich <jbeulich@xxxxxxxx>]

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).

> If this is a problem we could also change those options to be default
> to _off_ by renaming them to config DISABLE_xxxx

Yes, this would be a possibility, albeit not necessarily
one I would like.

Jan