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

[Julien Grall <julien@xen.org>]

Hi Stefano,

On 25/01/2021 21:27, Stefano Stabellini wrote:
>   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

I read through the back and forth between Bertrand and Jan about 
"UNSUPPORTED". However, I still don't understand why those two options 
are moved to UNSUPPORTED.

Both options will only build the code to enable the mitigation. The 
decision is still based on the processor you are running on.

In addition to that, ARM_SSBD can also be forced enabled/disabled on the 
command line.

A user may want to compile out the code if the target processor is not 
the affected by the two issues. This wouldn't be much different to Xen 
deciding to not enabling the mitigation.

I would view the two options as supported but not security supported. So 
this seems to fit exactly in the definition of EXPERT rather than 
UNSUPPORTED.

Cheers,

-- 
Julien Grall