Re: [PATCH v2 1/4] xen/arm: mpu: Introduce choice between MMU and MPU

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi,

> On 19 Sep 2024, at 14:33, Julien Grall <julien@xxxxxxx> wrote:
> 
> Hi Ayan,
> 
> On 18/09/2024 19:50, Ayan Kumar Halder wrote:
>> There are features in the forthcoming patches which are dependent on
>> MPU. For eg fixed start address.
>> Also, some of the Xen features (eg STATIC_MEMORY) will be selected
>> by the MPU configuration.
>> Thus, this patch introduces a choice between MMU and MPU for the type
>> of memory management system. By default, MMU is selected.
>> MPU is now gated by UNSUPPORTED.
>> Updated SUPPORT.md to state that the support for MPU is experimental.
>> Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
>> ---
>> Changes from :-
>> v1 - 1. Reworded the help messages.
>> 2. Updated Support.md.
>>  SUPPORT.md                     |  1 +
>>  xen/arch/arm/Kconfig           | 16 +++++++++++++++-
>>  xen/arch/arm/platforms/Kconfig |  2 +-
>>  3 files changed, 17 insertions(+), 2 deletions(-)
>> diff --git a/SUPPORT.md b/SUPPORT.md
>> index 23dd7e6424..3f6d788a43 100644
>> --- a/SUPPORT.md
>> +++ b/SUPPORT.md
>> @@ -40,6 +40,7 @@ supported in this document.
>>        Status, Xen in AArch64 mode: Supported
>>      Status, Xen in AArch32 mode: Tech Preview
>> +    Status, Xen with MPU: Experimental
>>      Status, Cortex A57 r0p0-r1p1: Supported, not security supported
>>      Status, Cortex A77 r0p0-r1p0: Supported, not security supported
>>  diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
>> index 323c967361..e881f5ba57 100644
>> --- a/xen/arch/arm/Kconfig
>> +++ b/xen/arch/arm/Kconfig
>> @@ -58,10 +58,24 @@ config PADDR_BITS
>>   default 40 if ARM_PA_BITS_40
>>   default 48 if ARM_64
>>  +choice
>> + prompt "Memory management system"
>> + default MMU if ARM
>> + help
>> +   User can choose between the different forms of memory management system.
>> +
>>  config MMU
>> - def_bool y
>> + bool "MMU"
>>   select HAS_PMAP
>>   select HAS_VMAP
>> + help
>> +   Select it if you plan to run Xen on A-profile Armv7+
>> +
>> +config MPU
>> + bool "MPU" if UNSUPPORTED
>> + help
>> +   Memory protection unit is supported on some Armv8-R systems 
>> (UNSUPPORTED).
> 
> I am a bit confused with this statement. Does this mean that not all Armv8-R 
> supports MPU? Or are you trying to say that not all of them support EL2?
> 
> If the former, we probably want to check pretty early during boot that the an 
> MPU is present *and* we have enough regions.

On Armv8-R if there is EL2 then there is an MPU at EL2.
Only EL1 might have an MMU with the MPU (but i do not think it can have only an 
MMU).

Cheers
Bertrand

> 
>> +endchoice
>>    source "arch/Kconfig"
>>  diff --git a/xen/arch/arm/platforms/Kconfig b/xen/arch/arm/platforms/Kconfig
>> index 76f7e76b1b..02322c259c 100644
>> --- a/xen/arch/arm/platforms/Kconfig
>> +++ b/xen/arch/arm/platforms/Kconfig
>> @@ -1,5 +1,5 @@
>>  choice
>> - prompt "Platform Support"
>> + prompt "Platform Support" if MMU
>>   default ALL_PLAT
>>   help
>>   Choose which hardware platform to enable in Xen.
> 
> Cheers,
> 
> -- 
> Julien Grall