Re: [PATCH v5 03/10] x86: Replace arch-specific boot_domain with the common one

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.07.2025 17:09, Alejandro Vallejo wrote:
> On Wed Jul 2, 2025 at 3:15 PM CEST, Jan Beulich wrote:
>> On 01.07.2025 12:56, Alejandro Vallejo wrote:
>>> --- a/xen/arch/x86/include/asm/bootfdt.h
>>> +++ b/xen/arch/x86/include/asm/bootfdt.h
>>> @@ -3,6 +3,12 @@
>>>  #define X86_BOOTFDT_H
>>>  
>>>  #include <xen/types.h>
>>> +#include <public/xen.h>
>>> +
>>> +struct arch_boot_domain
>>> +{
>>> +    domid_t domid;
>>> +};
>>>  
>>>  struct arch_boot_module
>>>  {
>>> [...]
>>> @@ -1048,11 +1050,11 @@ static struct domain *__init create_dom0(struct 
>>> boot_info *bi)
>>>          dom0_cfg.flags |= XEN_DOMCTL_CDF_iommu;
>>>  
>>>      /* Create initial domain.  Not d0 for pvshim. */
>>> -    bd->domid = get_initial_domain_id();
>>> -    d = domain_create(bd->domid, &dom0_cfg,
>>> +    bd->arch.domid = get_initial_domain_id();
>>> +    d = domain_create(bd->arch.domid, &dom0_cfg,
>>>                        pv_shim ? 0 : CDF_privileged | CDF_hardware);
>>>      if ( IS_ERR(d) )
>>> -        panic("Error creating d%u: %ld\n", bd->domid, PTR_ERR(d));
>>> +        panic("Error creating d%u: %ld\n", bd->arch.domid, PTR_ERR(d));
>>
>> This being the only place where the (now) arch-specific field is used, why
>> does it exist? A local variable would do? And if it's needed for
>> (supposedly arch-agnostic) hyperlaunch, then it probably shouldn't be
>> arch-specific? Daniel, Jason?
> 
> As for the arch-agnostic side of things, arm needs some extra work to be
> able to do it safely. dom0less currently constructs domains immediately after
> parsing them, which is problematic for cases where some domains have the prop
> and others don't. The domid allocation strategy may preclude further otherwise
> good domains from being created just because their domid was stolen by a 
> domain
> that didn't actually care about which domid it got.
> 
> It'll eventually want to leave the arch-specific area, but I don't want to do
> that work now.

But if the domU field is fine to live in a common struct despite being unused
on x86, why can't the domid field live in a common struct too, despite being
unused on non-x86? Otherwise it'll be extra churn for no gain to later move it
there.

Jan