Re: [PATCH v4 02/13] xen/arm: Introduce 'choice' for memory system architecture

[Julien Grall <julien@xxxxxxx>]

Hi Henry,

On 01/08/2023 04:44, Henry Wang wrote:
> There are two types of memory system architectures available for
> Arm-based systems, namely the Virtual Memory System Architecture (VMSA)
> and the Protected Memory System Architecture (PMSA). According to
> ARM DDI 0487G.a, A VMSA provides a Memory Management Unit (MMU) that
> controls address translation, access permissions, and memory attribute
> determination and checking, for memory accesses made by the PE. And
> refer to ARM DDI 0600A.c, the PMSA supports a unified memory protection
> scheme where an Memory Protection Unit (MPU) manages instruction and
> data access. Currently, Xen only suuports VMSA.
>
> As a preparation of the Xen MPU (PMSA) support. Introduce a Kconfig
> choice under the "Architecture Features" menu for user to choose the
> memory system architecture for the system. Since currently only VMSA
> is supported, only add the bool CONFIG_HAS_MMU to keep consistent with
> the default behavior. User can choose either VMSA or PMSA but not both
> in the future after PMSA/MPU is supported in Xen.
So in the long run I agree that we will want to have a choice. But this 
seems to be a bit premature to introduce it now as the user can't select 
the MPU and also can't deselect MMU.
Therefore, I think it would be best if we introduce an unselectable 
config for now. Like:
HAS_MMU
  def_bool y

This could be turned to a choice once you introduce the MPU.

Also, from my understanding, we are using the prefix HAS_ to indicate if 
an architecture support the given feature. In your case, you will want 
the user to select it, so I would just name the config MMU.
Cheers,

--
Julien Grall