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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan