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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.01.2021 22:27, Stefano Stabellini wrote:
> A recent thread [1] has exposed a couple of issues with our current way
> of handling EXPERT.
> 
> 1) It is not obvious that "Configure standard Xen features (expert
> users)" is actually the famous EXPERT we keep talking about on xen-devel
> 
> 2) It is not obvious when we need to enable EXPERT to get a specific
> feature
> 
> In particular if you want to enable ACPI support so that you can boot
> Xen on an ACPI platform, you have to enable EXPERT first. But searching
> through the kconfig menu it is really not clear (type '/' and "ACPI"):
> nothing in the description tells you that you need to enable EXPERT to
> get the option.
> 
> So this patch makes things easier by doing two things:
> 
> - introduce a new kconfig option UNSUPPORTED which is clearly to enable
>   UNSUPPORTED features as defined by SUPPORT.md
> 
> - change EXPERT options to UNSUPPORTED where it makes sense: keep
>   depending on EXPERT for features made for experts
> 
> - tag unsupported features by adding (UNSUPPORTED) to the one-line
>   description
> 
> - clarify the EXPERT one-line description
> 
> [1] https://marc.info/?l=xen-devel&m=160333101228981
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>

Non-Arm bits
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
However, I have questions on the Arm ones (sorry for not noticing
earlier, as I assume it was this way before already):

> @@ -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?

Jan