Included a software guideline for IOMMU emulation.

src/iommu_preface.adoc

@@ -40,6 +40,7 @@ and corrections, have been made since version 1.0.0:
 * Shadow stack encodings introduced by Zicfiss are reserved for IOMMU use.
 * Listed the fault codes reported for faults detected by Page Request.
 * Updated Fig 31 to remove the unused Destination ID field for ATS.PRGR
+* Included a software guideline for IOMMU emulation.
 
 These changes were made through PR#243 cite:[PR243].
 

src/iommu_sw_guidelines.adoc

@@ -319,6 +319,18 @@ Device-context whose `V` (valid) bit is clear, non-leaf PDT entries whose `V`
 PTEs whose `V` bit is clear. Software need not perform invalidations when
 changing the `V` bit in these entries from 0 to 1.
 
+==== Guidelines for emulating an IOMMU
+
+Certain uses may involve emulating a RISC-V IOMMU. In such cases, the emulator
+may require the IOMMU driver to notify the emulator for efficient operation when
+updates are made to in-memory data structure entries, including when making such
+entries valid. Queueing an appropriate invalidation command when making such
+updates is a common way to provide notifications to the emulator. While usually
+an invalidation is not required when marking an invalid entry as valid, the
+emulator may indicate the need to invoke such invalidation commands for
+emulation efficiency purposes through a suitable flag in the device tree or ACPI
+table describing such emulated IOMMU instances.
+
 === Reconfiguring PMAs
 
 Where platforms support dynamic reconfiguration of PMAs, a machine-mode driver