Re: [PATCH 01/23] xen: introduce hardware domain create flag

[Jason Andryuk <jason.andryuk@xxxxxxx>]

On 2025-03-07 11:26, Andrew Cooper wrote:
> On 07/03/2025 2:55 pm, Jason Andryuk wrote:
> > On 2025-03-06 17:39, Andrew Cooper wrote:
> > > Second, you've created a case where we can make multiple hardware
> > > domains, yet it is very much a singleton object from Xen's point of
> > > view.
> > hardware_domain still remains the check for is_hardware_domain(), so
> > it's still a singleton.
> Multiple domains can pass in CDF_hardware and latest-takes-precedence
> for hardware_domain.
>
> That only exists because late_hwdom is a bodge and relies on stealing.
>
> > A later ARM patch for the dom0less code adds a panic() if the device
> > tree defines a second hardware domains.
> Another option might be to strip out late_hwdom, and do this more
> nicely.  I have little confidence that it works, seeing as it only gets
> touched to fix build issues.
I don't want late_hwdom to hold up ARM side changes.  I'm not using 
late_hwdom, so I'd be fine removing it.  But until Hyperlaunch is 
merged, removal seems a little premature.
> Either way, I think the common code wants to be ultimately responsible
> for refusing to create multiple hardware domains.
>
> > > But, by the end, I think we do need to have reasonable confidence that
> > > only a single domain can be constructed as the hardware domain.
Would just an addition check be okay?  Only allow the late_hwdom 
behaviour for dom0.
--- i/xen/common/domain.c
+++ w/xen/common/domain.c
@@ -704,6 +704,10 @@ struct domain *domain_create(domid_t domid,
         if ( hardware_domid < 0 || hardware_domid >= 
DOMID_FIRST_RESERVED )
             panic("The value of hardware_dom must be a valid domain 
ID\n");
+        /* late_hwdom is only allowed for dom0. */
+        if ( hardware_domain && hardware_domain->domain_id )
+            return -EINVAL;
+
         old_hwdom = hardware_domain;
         hardware_domain = d;
     }

CDF_hardware is a Xen-internal flag, so it's not something the toolstack 
can pass in.
Regards,
Jason