diff --git a/arch/x86/pci/i386.c b/arch/x86/pci/i386.c
index 5525309..fe866c8 100644
--- a/arch/x86/pci/i386.c
+++ b/arch/x86/pci/i386.c
@@ -37,6 +37,7 @@
 #include <asm/pci_x86.h>
 #include <asm/io_apic.h>
 
+#define ALIGN_DOWN(x, a)	((x) & ~(a - 1))
 
 static int
 skip_isa_ioresource_align(struct pci_dev *dev) {
@@ -65,16 +66,21 @@ pcibios_align_resource(void *data, const struct resource *res,
 			resource_size_t size, resource_size_t align)
 {
 	struct pci_dev *dev = data;
-	resource_size_t start = res->start;
+	resource_size_t start = ALIGN_DOWN(res->end - size + 1, align);
 
 	if (res->flags & IORESOURCE_IO) {
-		if (skip_isa_ioresource_align(dev))
-			return start;
-		if (start & 0x300)
-			start = (start + 0x3ff) & ~0x3ff;
+
+		/*
+		 * If we're avoiding ISA aliases, the largest contiguous I/O
+		 * port space is 256 bytes.  Clearing bits 9 and 10 preserves
+		 * all 256-byte and smaller alignments, so the result will
+		 * still be correctly aligned.
+		 */
+		if (!skip_isa_ioresource_align(dev))
+			start &= ~0x300;
 	} else if (res->flags & IORESOURCE_MEM) {
 		if (start < BIOS_END)
-			start = BIOS_END;
+			start = res->end;	/* fail; no space */
 	}
 	return start;
 }