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

[Grygorii Strashko <grygorii_strashko@xxxxxxxx>]

On 23.07.25 12:16, Julien Grall wrote:
> Hi,
>
> On 23/07/2025 08:58, Grygorii Strashko wrote:
> > From: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
> >
> > 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@xxxxxxxx>
> > ---
> >   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