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Re: [PATCH] xen/arm: XEN_DOMCTL_get_address_size hypercall support
- To: Demi Marie Obenour <demiobenour@xxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Milan Djokic <milan_djokic@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
- Date: Fri, 3 Oct 2025 12:00:28 +0200
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 03 Oct 2025 10:00:57 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Thu Oct 2, 2025 at 8:27 PM CEST, 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@xxxxxxxx>
>>>
>>> ---
>>> 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?
HVM/PVH x86_64 cannot not be multiarch. All APs start in 16bit and proceed to
32 and 64 bit mode, in that order.
It's only PV that has fixed bitness.
Cheers,
Alejandro
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