@@ -75,3 +75,16 @@ info structure that's passed at boot time (field rsdp_paddr).
Description of paravirtualized devices will come from XenStore, just as it's
done for HVM guests.
+
+## VCPU hotplug ##
+
+VCPU hotplug (e.g. 'xl vcpu-set <domain> <num_vcpus>') for PVHv2 guests
+follows ACPI model where change in domain's number of VCPUS (stored in
+domain.avail_vcpus) results in an SCI being sent to the guest. The guest
+then executes DSDT's PRSC method, updating MADT enable status for the
+affected VCPU.
+
+This is achieved by having the toolstack issue a write to ACPI's
+XEN_ACPI_CPU_MAP (thus updating the VCPU available map stored there),
+followed by a write to ACPI GPE0 status register, setting XEN_GPE0_CPUHP_BIT.
+The latter will cause an SCI to be generated.