A later patch will upload LAPIC contexts as part of domain creation. In
order for it not to encounter a problem where the architectural state
does not reflect the APIC ID in the hidden state, this patch ensures
updates to the hidden state trigger an update in the architectural
registers so the APIC ID in both is consistent.

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
---
v7:
 * Rework the commit message so it explains a follow-up patch rather
   than hypothetical behaviour.
---
 xen/arch/x86/hvm/vlapic.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/xen/arch/x86/hvm/vlapic.c b/xen/arch/x86/hvm/vlapic.c
index 33b463925f4e..03581eb33812 100644
--- a/xen/arch/x86/hvm/vlapic.c
+++ b/xen/arch/x86/hvm/vlapic.c
@@ -1640,7 +1640,27 @@ static int cf_check lapic_load_hidden(struct domain *d, hvm_domain_context_t *h)
 
     s->loaded.hw = 1;
     if ( s->loaded.regs )
+    {
+        /*
+         * We already processed architectural regs in lapic_load_regs(), so
+         * this must be a migration. Fix up inconsistencies from any older Xen.
+         */
         lapic_load_fixup(s);
+    }
+    else
+    {
+        /*
+         * We haven't seen architectural regs so this could be a migration or a
+         * plain domain create. In the domain create case it's fine to modify
+         * the architectural state to align it to the APIC ID that was just
+         * uploaded and in the migrate case it doesn't matter because the
+         * architectural state will be replaced by the LAPIC_REGS ctx later on.
+         */
+        if ( vlapic_x2apic_mode(s) )
+            set_x2apic_id(s);
+        else
+            vlapic_set_reg(s, APIC_ID, SET_xAPIC_ID(s->hw.x2apic_id));
+    }
 
     hvm_update_vlapic_mode(v);
 
-- 
2.47.0