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

[Alejandro Vallejo <agarciav@amd.com>]

On Wed Jul 2, 2025 at 5:15 PM CEST, Jan Beulich wrote:
> 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

Mostly out of tidiness. Otherwise it's hard to know which fields serve a purpose
where.

I genuinely forgot about the domU field. I'm more than happy to drop that arch
subfield and have domid in the main body of the struct, but I suspect MISRA
would have something to say about dead data?

Cheers,
Alejandro