mm: Enable a dedicated kernel heap on BUILD_FLAT via MM_KERNEL_HEAP

Signed-off-by: Gustavo Henrique Nihei <gustavo.nihei@espressif.com>

include/nuttx/arch.h

@@ -644,6 +644,9 @@ void up_allocate_heap(FAR void **heap_start, size_t *heap_size);
  *   as determined by CONFIG_MM_KERNEL_HEAP=y.  This function allocates (and
  *   protects) the kernel-space heap.
  *
+ *   For Flat build (CONFIG_BUILD_FLAT=y), this function enables a separate
+ *   (although unprotected) heap for the kernel.
+ *
  ****************************************************************************/
 
 #ifdef CONFIG_MM_KERNEL_HEAP

include/nuttx/mm/mm.h

@@ -86,10 +86,16 @@
 #  define MM_KERNEL_USRHEAP_INIT 1
 #endif
 
-/* The kernel heap is never accessible from user code */
+/* When building the Userspace image under CONFIG_BUILD_KERNEL or
+ * CONFIG_BUILD_PROTECTED (i.e. !defined(__KERNEL__)), CONFIG_MM_KERNEL_HEAP
+ * must be undefined to ensure the kernel heap is never accessible from user
+ * code.
+ */
 
-#ifndef __KERNEL__
-#  undef CONFIG_MM_KERNEL_HEAP
+#if defined(CONFIG_BUILD_KERNEL) || defined(CONFIG_BUILD_PROTECTED)
+#  ifndef __KERNEL__
+#    undef CONFIG_MM_KERNEL_HEAP
+#  endif
 #endif
 
 /****************************************************************************

mm/Kconfig

@@ -26,16 +26,24 @@ config MM_CUSTOMIZE_MANAGER
 endchoice
 
 config MM_KERNEL_HEAP
-	bool "Support a protected, kernel heap"
-	default y
-	depends on !BUILD_FLAT
+	bool "Kernel dedicated heap"
+	default n if BUILD_FLAT
+	default y if BUILD_PROTECTED || BUILD_KERNEL
 	---help---
-		Partition heap memory into two parts:  (1) a protected, kernel-mode
-		heap accessible only by the NuttX kernel, and (2) an unprotected
-		user-mode heap for use by applications.  If you are only interested
-		in protected the kernel from read access, then this option is not
-		necessary.  If you wish to secure the kernel data as well, then
-		this option should be selected.
+		Under Flat build, this option will enable a separate heap for the kernel.
+		By separating the kernel and userspace heaps, the user is granted more
+		control over the heaps placement within the memory hierarchy, which is
+		specially useful for microcontrollers that provide External RAM.
+		Besides segregating the kernel and userspace allocations, this feature
+		does not prevent the userspace from accessing the kernel heap.
+
+		As for Protected and Kernel builds, this feature partitions heap memory
+		into two parts:
+		(1) a protected, kernel-mode heap accessible only by the NuttX kernel,
+		and (2) an unprotected user-mode heap for use by applications.
+		If you are only interested in protecting the kernel from read access,
+		then this option is not necessary. If you wish to secure the kernel data
+		as well, then this option should be selected.
 
 		The kernel heap size that is used is provided a a platform-specific
 		up_allocate_kheap() interface.  This configuration setting is made

mm/umm_heap/umm_initialize.c

@@ -43,8 +43,10 @@
  *
  *   CONFIG_BUILD_FLAT:
  *     There is only kernel mode "blob" containing both kernel and
- *     application code.  There is only one heap that is used by both the
- *     kernel and application logic.
+ *     application code. Depending upon the setting of CONFIG_MM_KERNEL_HEAP
+ *     there may be a single shared heap (used by both the kernel and
+ *     application logic) or there may be separate (although unprotected)
+ *     kernel and user heaps.
  *
  *     In this configuration, this function is called early in nx_start()
  *     to initialize the common heap.

sched/group/Make.defs

@@ -48,7 +48,7 @@ ifeq ($(CONFIG_BINFMT_LOADABLE),y)
 CSRCS += group_exitinfo.c
 endif
 
-ifneq ($(CONFIG_BUILD_FLAT),y)
+ifeq ($(CONFIG_MM_KERNEL_HEAP),y)
 CSRCS += group_malloc.c group_realloc.c group_zalloc.c group_free.c
 endif