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

[George Dunlap <gwd@xxxxxxxxxxxxxx>]

On Mon, Nov 2, 2026 at 9:50 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> 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.

This has beeen done.  It required some refactoring of later patches.
These have all been tested and skimmed, but haven't been over with a
fine-tooth comb again, so there's a risk of LLM quirks.  Do you mind
if I post what I have now, so people can look at it this week if the
chance affords?  Otherwise I won't be able to post it until next week
at the earliest.

 -George