It's observed that Linux kernel booting with the VM reports a "conflicting
mapping for input ID" FW_BUG.

The IORT doc defines "Number of IDs" to be "the number of IDs in the range
minus one", while virt-acpi-build.c simply stores the number of IDs in the
id_count without the "minus one". Meanwhile, some of the callers pass in a
0xFFFF following the spec. So, this is a mismatch between the function and
its callers.

Fix build_iort_id_mapping() by internally subtracting one from the pass-in
@id_count. Accordingly make sure that all existing callers pass in a value
without the "minus one", i.e. change all 0xFFFFs to 0x10000s.

Also, add a few lines of comments to highlight this change along with the
referencing document for this build_iort_id_mapping().

Fixes: 42e0f050e3a5 ("hw/arm/virt-acpi-build: Add IORT support to bypass SMMUv3")
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
---
Changelog
v4:
 * Rephrased the function documentation and used the latest IORT spec ver.
 * Added "Reviewed-by" from Eric
v3:
 https://lore.kernel.org/all/20240618211110.922809-1-nicolinc@nvidia.com/
 * Added "-1" internally in build_iort_id_mapping() instead
 * Added comments to highlight this and referencing doc
v2:
 https://lore.kernel.org/all/20240617223945.906996-1-nicolinc@nvidia.com/
 * Moved "-1" to the same line of id_count calculation
 * Added "+1" to the next_range.input_base calculation
v1:
 https://lore.kernel.org/all/20240613234802.828265-1-nicolinc@nvidia.com/

 hw/arm/virt-acpi-build.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
index c3ccfef026..60a79b91ca 100644
--- a/hw/arm/virt-acpi-build.c
+++ b/hw/arm/virt-acpi-build.c
@@ -209,12 +209,19 @@ static void acpi_dsdt_add_tpm(Aml *scope, VirtMachineState *vms)
 #define ROOT_COMPLEX_ENTRY_SIZE 36
 #define IORT_NODE_OFFSET 48
 
+/*
+ * Append an ID mapping entry as described by "Table 4 ID mapping format" in
+ * "IO Remapping Table System Software on ARM Platforms", Chapter 3.
+ * Document number: ARM DEN 0049E.f, Apr 2024
+ *
+ * Note that @id_count gets internally subtracted by one, following the spec.
+ */
 static void build_iort_id_mapping(GArray *table_data, uint32_t input_base,
                                   uint32_t id_count, uint32_t out_ref)
 {
-    /* Table 4 ID mapping format */
     build_append_int_noprefix(table_data, input_base, 4); /* Input base */
-    build_append_int_noprefix(table_data, id_count, 4); /* Number of IDs */
+    /* Number of IDs - The number of IDs in the range minus one */
+    build_append_int_noprefix(table_data, id_count - 1, 4);
     build_append_int_noprefix(table_data, input_base, 4); /* Output base */
     build_append_int_noprefix(table_data, out_ref, 4); /* Output Reference */
     /* Flags */
@@ -306,8 +313,8 @@ build_iort(GArray *table_data, BIOSLinker *linker, VirtMachineState *vms)
         }
 
         /* Append the last RC -> ITS ID mapping */
-        if (next_range.input_base < 0xFFFF) {
-            next_range.id_count = 0xFFFF - next_range.input_base;
+        if (next_range.input_base < 0x10000) {
+            next_range.id_count = 0x10000 - next_range.input_base;
             g_array_append_val(its_idmaps, next_range);
         }
 
@@ -366,7 +373,7 @@ build_iort(GArray *table_data, BIOSLinker *linker, VirtMachineState *vms)
         build_append_int_noprefix(table_data, 0, 4);
 
         /* output IORT node is the ITS group node (the first node) */
-        build_iort_id_mapping(table_data, 0, 0xFFFF, IORT_NODE_OFFSET);
+        build_iort_id_mapping(table_data, 0, 0x10000, IORT_NODE_OFFSET);
     }
 
     /* Table 17 Root Complex Node */
@@ -419,7 +426,7 @@ build_iort(GArray *table_data, BIOSLinker *linker, VirtMachineState *vms)
         }
     } else {
         /* output IORT node is the ITS group node (the first node) */
-        build_iort_id_mapping(table_data, 0, 0xFFFF, IORT_NODE_OFFSET);
+        build_iort_id_mapping(table_data, 0, 0x10000, IORT_NODE_OFFSET);
     }
 
     acpi_table_end(linker, &table);
-- 
2.43.0