powerpc/8xx: Reduce default size of module/execmem area

8xx boards don't have much memory, the two I know have respectively
32Mbytes and 128Mbytes, so there is no point in having 256 Mbytes of
memory for module text.

Reduce it to 32Mbytes for 8xx, that's more than enough.

Nevertheless, make it a configurable value so that it can be customised
if needed.

Also add a build verification for overlap of module execmem space
with user PMD.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/8db23b61e33a0d1913d814f94bfe71ba7ac78b0f.1724173828.git.christophe.leroy@csgroup.eu

arch/powerpc/Kconfig

@@ -1271,6 +1271,24 @@ config TASK_SIZE
 	default "0x80000000" if PPC_8xx
 	default "0xb0000000" if PPC_BOOK3S_32
 	default "0xc0000000"
+
+config MODULES_SIZE_BOOL
+	bool "Set custom size for modules/execmem area"
+	depends on EXECMEM && ADVANCED_OPTIONS
+	depends on PPC_8xx
+	help
+	  This option allows you to set the size of kernel virtual address
+	  space dedicated for modules/execmem.
+	  For the time being it is only for 8xx.
+
+	  Say N here unless you know what you are doing.
+
+config MODULES_SIZE
+	int "Size of modules/execmem area (In Mbytes)" if MODULES_SIZE_BOOL
+	range 1 256 if EXECMEM
+	default 32 if EXECMEM && PPC_8xx
+	default 0
+
 endmenu
 
 if PPC64

arch/powerpc/include/asm/nohash/32/mmu-8xx.h

@@ -170,8 +170,9 @@
 
 #define mmu_linear_psize	MMU_PAGE_8M
 
-#define MODULES_VADDR	(PAGE_OFFSET - SZ_256M)
 #define MODULES_END	PAGE_OFFSET
+#define MODULES_SIZE	(CONFIG_MODULES_SIZE * SZ_1M)
+#define MODULES_VADDR	(MODULES_END - MODULES_SIZE)
 
 #ifndef __ASSEMBLY__
 

arch/powerpc/mm/nohash/8xx.c

@@ -207,6 +207,8 @@ void __init setup_initial_memory_limit(phys_addr_t first_memblock_base,
 
 	/* 8xx can only access 32MB at the moment */
 	memblock_set_current_limit(min_t(u64, first_memblock_size, SZ_32M));
+
+	BUILD_BUG_ON(ALIGN_DOWN(MODULES_VADDR, PGDIR_SIZE) < TASK_SIZE);
 }
 
 int pud_clear_huge(pud_t *pud)