Fix memory update overlay to emit delta instead of full remove+add

Signed-off-by: Cong Wang <cwang@multikernel.io>

Author: congwang-mk

src/kerf/dtc/overlay.py

@@ -119,29 +119,53 @@ def generate_update_overlay(self, instance_name: str, old_instance, new_instance
         fdt_sw.begin_node("fragment@0")
         fdt_sw.begin_node("__overlay__")
 
-        # 1. memory-remove (if memory changed)
+        # 1. memory-remove (if memory shrunk or base changed)
         if memory_changed:
-            fdt_sw.begin_node("memory-remove")
-            fdt_sw.property_string("mk,instance", instance_name)
-
-            fdt_sw.begin_node("region@0")
-            reg_data = struct.pack(">QQ", old_mem_base, old_mem_size)
-            fdt_sw.property("reg", reg_data)
-            fdt_sw.end_node()
-
-            fdt_sw.end_node()
+            if old_mem_base == new_mem_base:
+                # Same base: only remove the excess if shrinking
+                if old_mem_size > new_mem_size:
+                    remove_base = old_mem_base + new_mem_size
+                    remove_size = old_mem_size - new_mem_size
+                    fdt_sw.begin_node("memory-remove")
+                    fdt_sw.property_string("mk,instance", instance_name)
+                    fdt_sw.begin_node("region@0")
+                    reg_data = struct.pack(">QQ", remove_base, remove_size)
+                    fdt_sw.property("reg", reg_data)
+                    fdt_sw.end_node()
+                    fdt_sw.end_node()
+            else:
+                # Different base: remove entire old region
+                fdt_sw.begin_node("memory-remove")
+                fdt_sw.property_string("mk,instance", instance_name)
+                fdt_sw.begin_node("region@0")
+                reg_data = struct.pack(">QQ", old_mem_base, old_mem_size)
+                fdt_sw.property("reg", reg_data)
+                fdt_sw.end_node()
+                fdt_sw.end_node()
 
-        # 2. memory-add (if memory changed)
+        # 2. memory-add (if memory grew or base changed)
         if memory_changed:
-            fdt_sw.begin_node("memory-add")
-            fdt_sw.property_string("mk,instance", instance_name)
-
-            fdt_sw.begin_node("region@0")
-            reg_data = struct.pack(">QQ", new_mem_base, new_mem_size)
-            fdt_sw.property("reg", reg_data)
-            fdt_sw.end_node()
-
-            fdt_sw.end_node()
+            if old_mem_base == new_mem_base:
+                # Same base: only add the extension if growing
+                if new_mem_size > old_mem_size:
+                    add_base = old_mem_base + old_mem_size
+                    add_size = new_mem_size - old_mem_size
+                    fdt_sw.begin_node("memory-add")
+                    fdt_sw.property_string("mk,instance", instance_name)
+                    fdt_sw.begin_node("region@0")
+                    reg_data = struct.pack(">QQ", add_base, add_size)
+                    fdt_sw.property("reg", reg_data)
+                    fdt_sw.end_node()
+                    fdt_sw.end_node()
+            else:
+                # Different base: add entire new region
+                fdt_sw.begin_node("memory-add")
+                fdt_sw.property_string("mk,instance", instance_name)
+                fdt_sw.begin_node("region@0")
+                reg_data = struct.pack(">QQ", new_mem_base, new_mem_size)
+                fdt_sw.property("reg", reg_data)
+                fdt_sw.end_node()
+                fdt_sw.end_node()
 
         # 3. cpu-remove (if CPUs removed)
         if cpus_to_remove:

src/kerf/update/main.py

@@ -280,13 +280,35 @@ def update_instance_operation(current):
 
             if memory_bytes is not None:
                 if memory_base_addr is None:
-                    found_base = find_available_memory_base(modified, memory_bytes)
-                    if found_base is None:
-                        raise ResourceError(
-                            f"No available memory region found for {memory_bytes} bytes. "
-                            "Try specifying --memory-base or reduce memory size."
-                        )
-                    memory_base_addr = found_base
+                    # Keep the same base address and extend/shrink in place
+                    old_base = existing_instance.resources.memory_base
+                    old_size = existing_instance.resources.memory_bytes
+
+                    if memory_bytes > old_size:
+                        # Growing: validate the extension region doesn't overlap
+                        extension_base = old_base + old_size
+                        extension_size = memory_bytes - old_size
+                        try:
+                            validate_memory_allocation(
+                                modified, extension_base, extension_size,
+                                exclude_instance=instance_node_name
+                            )
+                            memory_base_addr = old_base
+                        except (ResourceError,):
+                            # Extension conflicts, find a completely new region
+                            found_base = find_available_memory_base(modified, memory_bytes)
+                            if found_base is None:
+                                raise ResourceError(
+                                    f"No available memory region found for {memory_bytes} bytes. "
+                                    "Try specifying --memory-base or reduce memory size."
+                                )
+                            memory_base_addr = found_base
+                    elif memory_bytes < old_size:
+                        # Shrinking: always keep the same base
+                        memory_base_addr = old_base
+                    else:
+                        # Same size, no change
+                        memory_base_addr = old_base
                 else:
                     validate_memory_allocation(
                         modified, memory_base_addr, memory_bytes, exclude_instance=instance_node_name