Re: [PATCH v5 02/45] x86/mm: refuse to promote an L4 while the domain has no per-domain L3

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.10.2026 18:11, George Dunlap wrote:
> On Thu, Oct 29, 2026 at 7:47 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> Independent of that, how about pulling hvm_domain_initialise()'s
>> create_perdomain_mapping() up into arch_domain_create() instead of the
>> code additions you do?
> 
> So, let's see if we can solve it "properly".  Constraints:
> 
> - Eventually, not all domains will have a domain-wide L3; some will
> only have per-vCPU L3s.
> 
> - init_xen_l4_slots needs a domain always, but only needs a vcpu to
> get the L3 of vcpu_pt domains.
> 
> - PV vcpu_pt domains don't need an L3 installed after
> init_xen_l4_slots; they need to run on a clone of the L4 anyway.
> 
> So by the end of the series, we could arrange it like this:
> 
> non-vcpu_pt domains always allocate d->arch.l3_perdomain in
> arch_domain_create.  vcpu_pt domains always allocate
> v->arch.l3_perdomain in arch_vcpu_create.
> 
> Have __init_xen_l4_slots() take a domain and an L3; if the L3 is NULL,
> leave the perdomain slot empty.
> 
> Have init_xen_l4_slots() take a vcpu.  BUG_ON(is_hvm_vcpu(v) &&
> v->arch.asi.vcpu_pt && !v->arch.l3_perdomain), then pass the
> appropriate l3 into __init_xen_l4_slots().
> 
> Have init_xen_l4_slots_pv() take a domain, and always pass the domain
> and d->arch.l3_perdomain to __init_xen_l4_slots.  This will be NULL
> for PV vcpu_pt domains, but that's fine.
> 
> Then promote_l4_table() calls init_xen_l4_slots_pv() with a domain,
> and loses both the checks at the top.
> 
> There are corner cases to sort out, like how idle domains get an L3;
> but overall I think this makes everything more robust.
> 
> Thoughts?

Sounds plausible to me.

Jan