[PATCH 13/15] s390x/pci: Add a comment explaining S390PCIIOMMU purpose

[Konstantin Shkolnyy <kshk@linux.ibm.com>]

Signed-off-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
---
 include/hw/s390x/s390-pci-bus.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/hw/s390x/s390-pci-bus.h b/include/hw/s390x/s390-pci-bus.h
index e3cbee2695..2fab28e6e0 100644
--- a/include/hw/s390x/s390-pci-bus.h
+++ b/include/hw/s390x/s390-pci-bus.h
@@ -271,6 +271,14 @@ typedef struct S390PCIDMACount {
     QTAILQ_ENTRY(S390PCIDMACount) link;
 } S390PCIDMACount;
 
+/*
+ * This structure holds the PCI device AddressSpace that QEMU needs to link
+ * into its internal structures before the zPCI and PCI devices are fully
+ * initialized. It's a QEMU requirement to provide this "root" AddressSpace
+ * early. The AddressSpace is only actually used for I/O while the PCI
+ * device is plugged in and configured by the guest, at which time it gets
+ * additional memory subregions from zPCI device, that can do real work.
+ */
 struct S390PCIIOMMU {
     Object parent_obj;
     AddressSpace as;
-- 
2.34.1