Re: [PATCH] xen/arm: XEN_DOMCTL_get_address_size hypercall support

[Julien Grall <julien@xen.org>]

Hi Demi,

On 03/10/2025 17:03, Demi Marie Obenour wrote:
> On 10/3/25 06:14, Julien Grall wrote:
>> Hi Demi,
>>
>> On 02/10/2025 19:27, Demi Marie Obenour wrote:
>>> On 10/2/25 06:10, Andrew Cooper wrote:
>>>> On 01/10/2025 9:01 pm, Milan Djokic wrote:
>>>>> Signed-off-by: Milan Djokic <milan_djokic@epam.com>
>>>>>
>>>>> ---
>>>>> XEN_DOMCTL_get_address_size hypercall is not implemented for arm (only for x86)
>>>>> It would be useful to have this hypercall supported for arm64, in order to get
>>>>> current guest addressing mode and also to verify that XEN_DOMCTL_set_address_size
>>>>> performs switch to target addressing mode (instead of relying on its returned error code only).
>>>>
>>>> Please don't copy this misfeature of x86 PV guests into ARM.
>>>>
>>>> Letting domains be of variable bitness after domain create leads to a
>>>> whole lot of bugs, many security relevant.
>>>>
>>>> 32bit vs 64bit should be an input to domain_create(), not something that
>>>> is edited after the domain has been constructed.
>>>
>>> Does this mean that Xen guests cannot support multiarch?
>>
>> I can't speak for x86. But for Arm, the endianess of EL1 (OS) is fixed
>> when the vCPU is booting. You could in theory have a domain with a mix
>> of 64-bit and 32-bit vCPUs. But that's not supported by Xen (all vCPUs
>> should have the same bitness) and also I am not aware of any mainstream
>> OS able to deal with multiple bitness. Most likely, you will need to run
>> two OSes and create your custom OS.
>>
>> Also, I believe XEN_DOMCTL_get_address_size would not be suitable for
>> such setup.
> 
> I meant multiarch in userspace.  Running a 32-bit kernel makes no sense.

I think some people may disagree :). We are in process of adding support 
for 32-bit MPU in Xen.

> If that is something Arm OSs just don't support that's fine too.

HCR_EL2.RW is only forcing 64-bit in EL1 (kernel mode). It is still 
possible to run 32-bit applicate in userland as long as the process 
supports it.

Cheers,

-- 
Julien Grall