x86, bios: Make the x86 early memory reservation a kernel option

Add a kernel command-line option so the x86 early memory reservation
size can be adjusted at runtime instead of only at compile time.
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <tip-d0cd7425@git.kernel.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

Documentation/kernel-parameters.txt

@@ -2150,6 +2150,11 @@ and is between 256 and 4096 characters. It is defined in the file
 			Reserves a hole at the top of the kernel virtual
 			address space.
 
+	reservelow=	[X86]
+			Format: nn[K]
+			Set the amount of memory to reserve for BIOS at
+			the bottom of the address space.
+
 	reset_devices	[KNL] Force drivers to reset the underlying device
 			during initialization.
 

arch/x86/Kconfig

@@ -1326,7 +1326,7 @@ config X86_BOOTPARAM_MEMORY_CORRUPTION_CHECK
 	  Set whether the default state of memory_corruption_check is
 	  on or off.
 
-config X86_LOW_RESERVE
+config X86_RESERVE_LOW
 	int "Amount of low memory, in kilobytes, to reserve for the BIOS"
 	default 64
 	range 4 640

arch/x86/kernel/setup.c

@@ -618,6 +618,8 @@ static __init void reserve_ibft_region(void)
 		reserve_early_overlap_ok(addr, addr + size, "ibft");
 }
 
+static unsigned reserve_low = CONFIG_X86_RESERVE_LOW << 10;
+
 static void __init trim_bios_range(void)
 {
 	/*
@@ -627,9 +629,9 @@ static void __init trim_bios_range(void)
 	 *
 	 * This typically reserves additional memory (64KiB by default)
 	 * since some BIOSes are known to corrupt low memory.  See the
-	 * Kconfig help text for X86_LOW_RESERVE.
+	 * Kconfig help text for X86_RESERVE_LOW.
 	 */
-	e820_update_range(0, ALIGN(CONFIG_X86_LOW_RESERVE << 10, PAGE_SIZE),
+	e820_update_range(0, ALIGN(reserve_low, PAGE_SIZE),
 			  E820_RAM, E820_RESERVED);
 
 	/*
@@ -641,6 +643,28 @@ static void __init trim_bios_range(void)
 	sanitize_e820_map(e820.map, ARRAY_SIZE(e820.map), &e820.nr_map);
 }
 
+static int __init parse_reservelow(char *p)
+{
+	unsigned long long size;
+
+	if (!p)
+		return -EINVAL;
+
+	size = memparse(p, &p);
+
+	if (size < 4096)
+		size = 4096;
+
+	if (size > 640*1024)
+		size = 640*1024;
+
+	reserve_low = size;
+
+	return 0;
+}
+
+early_param("reservelow", parse_reservelow);
+
 /*
  * 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