Re: [PATCH] x86/hvm: Add APIC IDs to the per-vLAPIC save area

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Feb 26, 2025 at 02:11:23PM +0100, Jan Beulich wrote:
> On 18.02.2025 15:22, Alejandro Vallejo wrote:
> > Today, Xen hardcodes apic_id = vcpu_id * 2, but this is unwise and
> > interferes with providing accurate topology information to the guest.
> > 
> > Introduce a new x2apic_id field into hvm_hw_lapic.  This is immutable
> > state from the guest's point of view, but it will allow the toolstack to
> > eventually configure the value, and for the value to move on migrate.
> > 
> > For backwards compatibility, the patch rebuilds the old-style APIC IDs
> > from migration streams lacking them when they aren't present.
> 
> Nit: "when they aren't present" looks to duplicate "lacking them"?
> 
> > Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
> > ---
> > I've split this one from the rest of the topology series as it's independent
> > and entangled with another patch from Andrew.
> 
> Albeit I think meanwhile we've settled that the entangling isn't quite as
> problematic.
> 
> > @@ -1621,6 +1624,14 @@ static int cf_check lapic_load_hidden(struct domain *d, hvm_domain_context_t *h)
> >          return -EINVAL;
> >      }
> >  
> > +    /*
> > +     * Xen 4.20 and earlier had no x2APIC ID in the migration stream and
> > +     * hard-coded "vcpu_id * 2". Default back to this if we have a
> > +     * zero-extended record.
> > +     */
> > +    if ( h->size <= offsetof(struct hvm_hw_lapic, x2apic_id) )
> > +        s->hw.x2apic_id = v->vcpu_id * 2;
> 
> While we better wouldn't get to see such input, it is in principle possible
> to have an input stream with, say, half the field. Imo the condition ought
> to be such that we'd make the adjustment when less than the full field is
> available.

I would add an additional check to ensure _rsvd0 remains 0, to avoid
further additions from attempting to reuse that padding space.

if ( s->hw._rsvd0 )
    return -EINVAL;

In fact I would be tempted to overwrite the ID if the stream size
doesn't match the expected one, ie:

if ( h->size < (offsetof(struct hvm_hw_lapic, _rsvd0) +
                sizeof(s->hw._rsvd0)) )
    s->hw.x2apic_id = v->vcpu_id * 2;

Regards, Roger.