x86, mm: Move reserving low memory later in initialization

Move the reservation of low memory, except for the 4K which actually
does belong to the BIOS, later in the initialization; in particular,
after we have already reserved the trampoline.

The current code locates the trampoline as high as possible, so by
deferring the allocation we will still be able to reserve as much
memory as is possible.  This allows us to run with reservelow=640k
without getting a crash on system startup.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/n/tip-0y9dqmmsousf69wutxwl3kkf@git.kernel.org

arch/x86/kernel/setup.c

@@ -608,8 +608,6 @@ static __init void reserve_ibft_region(void)
 		memblock_reserve(addr, size);
 }
 
-static unsigned reserve_low = CONFIG_X86_RESERVE_LOW << 10;
-
 static bool __init snb_gfx_workaround_needed(void)
 {
 #ifdef CONFIG_PCI
@@ -698,8 +696,7 @@ static void __init trim_bios_range(void)
 	 * since some BIOSes are known to corrupt low memory.  See the
 	 * Kconfig help text for X86_RESERVE_LOW.
 	 */
-	e820_update_range(0, ALIGN(reserve_low, PAGE_SIZE),
-			  E820_RAM, E820_RESERVED);
+	e820_update_range(0, PAGE_SIZE, E820_RAM, E820_RESERVED);
 
 	/*
 	 * special case: Some BIOSen report the PC BIOS
@@ -711,6 +708,8 @@ static void __init trim_bios_range(void)
 	sanitize_e820_map(e820.map, ARRAY_SIZE(e820.map), &e820.nr_map);
 }
 
+static unsigned reserve_low = CONFIG_X86_RESERVE_LOW << 10;
+
 static int __init parse_reservelow(char *p)
 {
 	unsigned long long size;
@@ -733,6 +732,11 @@ static int __init parse_reservelow(char *p)
 
 early_param("reservelow", parse_reservelow);
 
+static void __init trim_low_memory_range(void)
+{
+	memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE));
+}
+	
 /*
  * Determine if we were loaded by an EFI loader.  If so, then we have also been
  * passed the efi memmap, systab, etc., so we should use these data structures
@@ -987,6 +991,7 @@ void __init setup_arch(char **cmdline_p)
 	setup_real_mode();
 
 	trim_platform_memory_ranges();
+	trim_low_memory_range();
 
 	init_gbpages();