Re: [XEN][PATCH 2/8] xen/arm: move vcpu_switch_to_aarch64_mode() in arch_vcpu_create()

[Grygorii Strashko <grygorii_strashko@epam.com>]

On 23.07.25 12:16, Julien Grall wrote:
> Hi,
> 
> On 23/07/2025 08:58, Grygorii Strashko wrote:
>> From: Grygorii Strashko <grygorii_strashko@epam.com>
>>
>> Move vcpu_switch_to_aarch64_mode() in arch_vcpu_create() callback instead
>> of calling it manually from few different places after vcpu_create().
>>
>> Before doing above ensure vcpu0 is created after kernel_probe() is done and
>> domain's guest execution mode (32-bit/64-bit) is set for dom0 and dom0less
>> domains.
> 
> The commit message doesn't mention anything about domains created by the toolstack. In this case, from my understanding, the switch to 64-bit domain happens *after* the vCPUs are created.
> 
> At the moment, I think this is probably ok to call...
> 
>>
>> Signed-off-by: Grygorii Strashko <grygorii_strashko@epam.com>
>> ---
>>   xen/arch/arm/domain.c                    |  3 +++
>>   xen/arch/arm/domain_build.c              | 13 +++++--------
>>   xen/common/device-tree/dom0less-build.c  |  6 +++---
>>   xen/include/asm-generic/dom0less-build.h |  2 +-
>>   4 files changed, 12 insertions(+), 12 deletions(-)
>>
>> diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
>> index 79a144e61be9..bbd4a764c696 100644
>> --- a/xen/arch/arm/domain.c
>> +++ b/xen/arch/arm/domain.c
>> @@ -586,6 +586,9 @@ int arch_vcpu_create(struct vcpu *v)
>>       if ( get_ssbd_state() == ARM_SSBD_RUNTIME )
>>           v->arch.cpu_info->flags |= CPUINFO_WORKAROUND_2_FLAG;
>> +    if ( is_64bit_domain(v->domain) )
>> +        vcpu_switch_to_aarch64_mode(v);
> 
> ... this function here because I *think* it would be NOP. But this feels really fragile.

The toolstack configures domain and vcpus through XEN_DOMCTL_set_address_size on Arm64:
- toolstack creates domain and parses kernel binary: domain created with DOMAIN_32BIT mode by default
- toolstack creates vcpus (still 32 bit mode): libxl__build_pre()->xc_domain_max_vcpus()
- toolstack switches domain mode depending on kernel binary type: libxl__build_dom()->xc_dom_boot_mem_init(),
   which triggers XEN_DOMCTL_set_address_size hypercall.
   Xen: arm64: switches domain mode and re-configures vcpus: subarch_do_domctl()->set_address_size()

So, this patch does not affect toolstack path, only optimizes Xen boots a bit.

Also, during Xen boot or by toolstack - the domain is always created before it's type is even known, which, in turn,
is based on guest binary which is parsed later during domain configuration stage.

I can add note in commit message "This patch doesn't affect on the toolstack Arm64 domain creation path as toolstack always
re-configures domain mode and vcpus through XEN_DOMCTL_set_address_size hypercall during domain configuration stage"

> 
> If the desire is to make 32-bit domain optional on Arm64. Then I think it would be better to pass the domain type when the domain
> is created (IOW add an extra flags to XEN_DOMCTL_createdomain). This will require more work, but it will be a lot more robust.



-- 
Best regards,
-grygorii