x86: avoid high BIOS area when allocating address space

This prevents allocation of the last 2MB before 4GB.

The experiment described here shows Windows 7 ignoring the last 1MB:
https://bugzilla.kernel.org/show_bug.cgi?id=23542#c27

This patch ignores the top 2MB instead of just 1MB because H. Peter Anvin
says "There will be ROM at the top of the 32-bit address space; it's a fact
of the architecture, and on at least older systems it was common to have a
shadow 1 MiB below."

Acked-by: H. Peter Anvin <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Signed-off-by: Jesse Barnes <[email protected]>

Author: Bjorn Helgaas

arch/x86/include/asm/e820.h

@@ -72,6 +72,9 @@ struct e820map {
 #define BIOS_BEGIN		0x000a0000
 #define BIOS_END		0x00100000
 
+#define BIOS_ROM_BASE		0xffe00000
+#define BIOS_ROM_END		0xffffffff
+
 #ifdef __KERNEL__
 /* see comment in arch/x86/kernel/e820.c */
 extern struct e820map e820;

arch/x86/kernel/resource.c

@@ -37,10 +37,11 @@ static void remove_e820_regions(struct resource *avail)
 
 void arch_remove_reservations(struct resource *avail)
 {
-	/* Trim out BIOS area (low 1MB) and E820 regions */
+	/* Trim out BIOS areas (low 1MB and high 2MB) and E820 regions */
 	if (avail->flags & IORESOURCE_MEM) {
 		if (avail->start < BIOS_END)
 			avail->start = BIOS_END;
+		resource_clip(avail, BIOS_ROM_BASE, BIOS_ROM_END);
 
 		remove_e820_regions(avail);
 	}