Re: [PATCH v4 6/6] xen/arm: mpu: Implement a dummy enable_secondary_cpu_mm

[Jan Beulich <jbeulich@suse.com>]

On 29.10.2024 10:30, Luca Fancellu wrote:
> Hi Jan,
> 
>> On 29 Oct 2024, at 08:08, Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 28.10.2024 18:38, Ayan Kumar Halder wrote:
>>> On 28/10/2024 15:01, Jan Beulich wrote:
>>>> On 28.10.2024 15:39, Ayan Kumar Halder wrote:
>>>>> On 28/10/2024 12:55, Jan Beulich wrote:
>>>>>> On 28.10.2024 13:45, Ayan Kumar Halder wrote:
>>>>>>> --- a/xen/arch/Kconfig
>>>>>>> +++ b/xen/arch/Kconfig
>>>>>>> @@ -6,11 +6,13 @@ config PHYS_ADDR_T_32
>>>>>>>
>>>>>>>   config NR_CPUS
>>>>>>>   	int "Maximum number of CPUs"
>>>>>>> +	range 1 1 if ARM && MPU
>>>>>>>   	range 1 16383
>>>>>>>   	default "256" if X86
>>>>>>>   	default "8" if ARM && RCAR3
>>>>>>>   	default "4" if ARM && QEMU
>>>>>>>   	default "4" if ARM && MPSOC
>>>>>>> +	default "1" if ARM && MPU
>>>>>>>   	default "128" if ARM
>>>>>>>   	help
>>>>>>>   	  Controls the build-time size of various arrays and bitmaps
>>>>>> I'm afraid I can't easily tell whether MPU can be used together with any of
>>>>>> RCAR3, QEMU, or MPSOC. If it can, the new default line would need to move
>>>>>> up, as it's the first one that has a match on its condition which is being
>>>>>> used.
>>>>> MPU cannot be used with any of the existing platforms.
>>>> That is - qemu can't emulate such an environment, i.e. even QEMU and MPU
>>>> don't go together?
>>>
>>> Qemu has support for Aarch32 MPU at EL2 and EL1 (ie R52). As far as I am 
>>> aware, there is no support for Aarch64 MPU in Qemu (ie R82).
>>>
>>> Even for R52, I could not get the upstream Qemu working (emulating some 
>>> Arm reference platform).
>>>
>>> I could get the Xilinx fork of Qemu (https://github.com/Xilinx/qemu) 
>>> working which emulates AMD's SoC using R52.
>>>
>>> However, this should not impact the current patch. There is no Qemu in 
>>> xen/arch/arm/platforms/*.
>>
>> Aiui that's not relevant. There is a QEMU item in xen/arch/arm/platforms/Kconfig.
>> I continue to fail to see why that couldn't be selected together with MPU. Yet if
>> it can be, you'd end up with a default of 4, not 1, if it actually _is_ selected.
>> Alternatively QEMU (and maybe also RCAR3 and MPSOC) need to be mutually exclusive
>> with MPU. Hmm, looks like that's already the case, by patch 2 suppressing the
>> "Platform Support" prompt. While that looks fragile to me, I'm sorry for the
>> noise then.
> 
> Are you suggesting to move "default "1" if ARM && MPU” right after “default "256" if X86”?

Yes.

Jan