[PATCH v8 13/16] s390x/pci: Add a comment explaining S390PCIIOMMU purpose

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

Now that S390PCIIOMMU had been reduced to only the IOMMU container
information needed by the PCI layer for the associated slot, add a
comment explaining the purpose of the structure.

Reviewed-by: Matthew Rosato <mjrosato@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..0a59be1fd0 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 AddressSpace for a PCI device slot. It must be
+ * allocated before PCIDevice registration completes, specifically to satisfy
+ * the get_address_space IOMMU callback invoked by do_pci_register_device().
+ * The root MemoryRegion is otherwise empty; DMA translations only become
+ * functional once the guest configures the device, at which point
+ * s390_pci_iommu_enable() registers the zPCI translation subregions beneath it.
+ */
 struct S390PCIIOMMU {
     Object parent_obj;
     AddressSpace as;
-- 
2.34.1