Re: [XEN][PATCH 2/7] xen/domctl: introduce XEN_DOMCTL_CDF_is_32bits

[Grygorii Strashko <grygorii_strashko@xxxxxxxx>]

On 31.07.25 18:19, Julien Grall wrote:
> Hi Teddy,
>
> On 31/07/2025 15:02, Teddy Astie wrote:
> > Hello,
> >
> > Le 31/07/2025 à 11:44, Grygorii Strashko a écrit :
> > > Hence, this patch introduces extra "XEN_DOMCTL_CDF_is_32bits" flag which is
> > > intended to be used by 64bit Arches for proper configuration of domain type
> > > when domain is created.
> > >
> > > Now it adds initial support for this flag for Arm64 arch only. The default
> > > Arm64 domain type is changed to 64bit:
> > > - the Arm Xen boot code is handling this case properly already;
> > > - for toolstack case the XEN_DOMCTL_set_address_size hypercall handling
> > > updated to forcibly configure domain type regardless of current domain type
> > > configuration. Hence toolstack configures vcpus and memory after
> > > configuring domain type it allows to start with domain default AArch64 type
> > > and then switch to requested domain type and ensures all required domain
> > > settings applied.
> > >
> > > For Arm32 this flag is ignored.
> > >
> > > Note. For Arm64, Once toolstack is updated to probe guest binary before
> > > creating domain the XEN_DOMCTL_set_address_size will become obsolete.
> > >
> > > [1] https://lists.xen.org/archives/html/xen-devel/2025-07/msg01647.html
> > > [2] https://lists.xen.org/archives/html/xen-devel/2025-07/msg01648.html
> > >
> > > Signed-off-by: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
> > > ---
> > I would prefer it be better presented as a platform-specific enum.
> > Something like
> >
> > ```
> > enum xen_arch_subarch {
> We need to be cautious when using enum in the public interface because, if I am 
> not mistaken, the size of the enum if not stable.
>
> >     XEN_ARM_SUBARCH_NATIVE = 0,
> "native" is a bit ambiguous when a platform is able to support 32-bit and 
> 64-bit. Does this refer to the bitness of the hypervisor (EL2)? Firmware (EL3)?
>
> That said, I am not entirely sure I see the value of native because EL1 (the 
> kernel exception level) has to be known at boot. So it is better to explicitly 
> mention whether the kernel is 32-bit or 64-bit.
>
> >     XEN_ARM_SUBARCH_ARMV7 = 1,
> This name would not be correct. You can have a 32-bit Armv8 guest. A better 
> name is SUBARCH_AARCH32.
>
> >     XEN_ARM_SUBARCH_AARCH64 = 2,
I've been thinking how about using ELF "e_machine" to identify guest?

It defines all arch ids already (no need to invent anything custom),
plus supports EM_NONE which can be treated as default value or indicate that
guest type is not set - arch specific handling.


config/domain structs:
   uint16_t e_machine;

Arch code can define and check number of supported ids, for example:
 arm: EM_NONE, EM_ARM (32bit), EM_AARCH64 (64bit)
 x86: EM_NONE [, EM_386 (32bit), EM_X86_64 (64bit) ]



--
Best regards,
-grygorii