Re: [PATCH v3 2/4] xen: make VMAP only support in MMU system

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.08.2024 19:13, Ayan Kumar Halder wrote:
> From: Penny Zheng <penny.zheng@xxxxxxx>
> 
> Introduced CONFIG_VMAP which is selected by the architectures that use
> MMU. vm_init() does not do anything if CONFIG_VMAP is not enabled.
> 
> VMAP is widely used in ALTERNATIVE feature to remap a range of memory
> with new memory attributes. Since this is highly dependent on virtual
> address translation, we choose to fold VMAP in MMU system.
> 
> In this patch, we introduce a new Kconfig CONFIG_HAS_VMAP, and make it
> only support in MMU system on ARM architecture. And ALTERNATIVE now
> depends on VMAP.
> 
> HARDEN_BRANCH_PREDICTOR is now gated on HAS_VMAP as speculative
> attacks are not possible on non MMU based systems (ie Cortex-R52, R82).
> See 
> https://developer.arm.com/Arm%20Security%20Center/Speculative%20Processor%20Vulnerability.

While I'm not an Arm expert and hence I'm likely missing aspects, I question
the one (Spectre-BHB) vulnerability there to be sufficient to draw a
conclusion towards the usefulness of branch hardening. I would advise
against encoding such a connection in the Kconfig dependencies.

> --- a/xen/arch/x86/Kconfig
> +++ b/xen/arch/x86/Kconfig
> @@ -1,6 +1,7 @@
>  config X86_64
>       def_bool y
>       select 64BIT
> +     select HAS_VMAP
>  
>  config X86
>       def_bool y
> @@ -31,6 +32,7 @@ config X86
>       select HAS_UBSAN
>       select HAS_VPCI if HVM
>       select NEEDS_LIBELF
> +     select HAS_VMAP

Why in two places? Also please respect alphabetic sorting here (if this
hunk is kept, which may be the more consistent approach) ...

> --- a/xen/common/Kconfig
> +++ b/xen/common/Kconfig
> @@ -77,6 +77,9 @@ config HAS_PIRQ
>  config HAS_PMAP
>       bool
>  
> +config HAS_VMAP
> +     bool
> +
>  config HAS_SCHED_GRANULARITY
>       bool

... and here.

Jan