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

[Julien Grall <julien@xxxxxxx>]

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@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?
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.
Cheers,

--
Julien Grall