Re: [PATCH v2 3/4] xen/arm64: allow to make aarch32 support optional

[Demi Marie Obenour <demiobenour@xxxxxxxxx>]

On 8/6/25 05:49, Grygorii Strashko wrote:
> From: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
> 
> Now Arm64 AArch32 guest support is always enabled and built-in while not
> all Arm64 platforms supports AArch32 (for exmaple on Armv9A) or this
> support might not be needed (Arm64 AArch32 is used quite rarely in embedded
> systems). More over, when focusing on safety certification, AArch32 related
> code in Xen leaves a gap in terms of coverage that cannot really be
> justified in words. This leaves two options: either support it (lots of
> additional testing, requirements and documents would be needed) or compile
> it out.
> 
> Hence, this patch introduces basic support to allow make Arm64
> AArch32 guest support optional. The following changes are introduced:
> 
> - Introduce Kconfig option CONFIG_ARM64_AARCH32 to allow enable/disable
>   Arm64 AArch32 guest support (default y)
> 
> - Introduce is_aarch32_enabled() helper which accounts Arm64 HW capability
>   and CONFIG_ARM64_AARCH32 setting
> 
> - Introduce arm64_set_domain_type() to configure Arm64 domain type in
>   unified way instead of open coding (d)->arch.type, and account
>   CONFIG_ARM64_AARCH32 configuration.
> 
> - toolstack: do not advertise "xen-3.0-armv7l " capability if AArch32 is
>   disabled.
> 
> - do not expose EL1 AArch32 support to guest in ID_AA64PFR0_EL1 reg if
>   AArch32 is disabled.
> 
> - Set Arm64 domain type to DOMAIN_64BIT by default.
>   - 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, so toolstack behavior is unchanged.
> 
> With CONFIG_ARM64_AARCH32=n the Xen will reject AArch32 guests (kernels) if
> configured by user in the following way:
> - Xen boot will fail with panic during dom0 or dom0less domains creation
> - toolstack domain creation will be rejected due to xc_dom_compat_check()
>   failure.
> 
> Making Arm64 AArch32 guest support open further possibilities for build
> optimizations of Arm64 AArch32 guest support code.
> 
> Signed-off-by: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
> ---
> changes in v2:
> - use Arm64 "cpu_has_el1_32" in all places to check if HW has AArch32 support
> - rework Arm64 XEN_DOMCTL_set_address_size hypercall handling to work with any
>   initial domain type set (32bit or 64 bit)
> - fix comments related to macro parameters evaluation issues
> - do not expose EL1 AArch32 support to guest in ID_AA64PFR0_EL1 reg if
>   AArch32 is disabled
> 
>  xen/arch/arm/Kconfig                    |  7 ++++
>  xen/arch/arm/arm64/domain-build.c       | 46 +++++++++++++++++++++++--
>  xen/arch/arm/arm64/domctl.c             | 16 +++++----
>  xen/arch/arm/arm64/vsysreg.c            |  9 +++++
>  xen/arch/arm/domain.c                   |  9 +++++
>  xen/arch/arm/domain_build.c             | 21 +++--------
>  xen/arch/arm/include/asm/arm32/domain.h | 12 +++++++
>  xen/arch/arm/include/asm/arm64/domain.h | 23 +++++++++++++
>  xen/arch/arm/setup.c                    |  2 +-
>  9 files changed, 119 insertions(+), 26 deletions(-)
> 
> diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
> index a0c816047427..bf6dd73caf73 100644
> --- a/xen/arch/arm/Kconfig
> +++ b/xen/arch/arm/Kconfig
> @@ -266,6 +266,13 @@ config PCI_PASSTHROUGH
>       help
>         This option enables PCI device passthrough
>  
> +config ARM64_AARCH32
> +     bool "AArch32 Guests support on on ARM64 (UNSUPPORTED)" if UNSUPPORTED
> +     depends on ARM_64
> +     default y
> +     help
> +       This option enables AArch32 Guests on ARM64.
> +
>  endmenu
Why UNSUPPORTED?  A safety-certified build of Xen should only be using
security-supported features.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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