RISC-V: Fix memory reservation in setup_bootmem()

Currently, the setup_bootmem() reserves memory from RAM start to the
kernel end. This prevents us from exploring ways to use the RAM below
(or before) the kernel start hence this patch updates setup_bootmem()
to only reserve memory from the kernel start to the kernel end.
Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/mm/init.c

@@ -21,6 +21,8 @@ unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]
 							__page_aligned_bss;
 EXPORT_SYMBOL(empty_zero_page);
 
+extern char _start[];
+
 static void __init zone_sizes_init(void)
 {
 	unsigned long max_zone_pfns[MAX_NR_ZONES] = { 0, };
@@ -95,18 +97,14 @@ void __init setup_bootmem(void)
 {
 	struct memblock_region *reg;
 	phys_addr_t mem_size = 0;
+	phys_addr_t vmlinux_end = __pa(&_end);
+	phys_addr_t vmlinux_start = __pa(&_start);
 
 	/* Find the memory region containing the kernel */
 	for_each_memblock(memory, reg) {
-		phys_addr_t vmlinux_end = __pa(_end);
 		phys_addr_t end = reg->base + reg->size;
 
 		if (reg->base <= vmlinux_end && vmlinux_end <= end) {
-			/*
-			 * Reserve from the start of the region to the end of
-			 * the kernel
-			 */
-			memblock_reserve(reg->base, vmlinux_end - reg->base);
 			mem_size = min(reg->size, (phys_addr_t)-PAGE_OFFSET);
 
 			/*
@@ -120,6 +118,9 @@ void __init setup_bootmem(void)
 	}
 	BUG_ON(mem_size == 0);
 
+	/* Reserve from the start of the kernel to the end of the kernel */
+	memblock_reserve(vmlinux_start, vmlinux_end - vmlinux_start);
+
 	set_max_mapnr(PFN_DOWN(mem_size));
 	max_low_pfn = PFN_DOWN(memblock_end_of_DRAM());
 
@@ -197,7 +198,6 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
 
 asmlinkage void __init setup_vm(void)
 {
-	extern char _start;
 	uintptr_t i;
 	uintptr_t pa = (uintptr_t) &_start;
 	pgprot_t prot = __pgprot(pgprot_val(PAGE_KERNEL) | _PAGE_EXEC);