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

[Henry Wang <Henry.Wang@xxxxxxx>]

Hi Julien,

> On Aug 9, 2023, at 20:04, Julien Grall <julien@xxxxxxx> wrote:
> 
> 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.

Make sense, I will just introduce a

MMU
  def_bool y

in this patch and turn this to a choice between CONFIG_MMU and CONFIG_MPU
in the last patch of the full MPU series where MPU is expected to be functional.

Kind regards,
Henry

> 
> Cheers,
> 
> -- 
> Julien Grall