diff --git a/arch/i386/pci/mmconfig.c b/arch/i386/pci/mmconfig.c
index 6b1ea0c9a570..e545b0992c48 100644
--- a/arch/i386/pci/mmconfig.c
+++ b/arch/i386/pci/mmconfig.c
@@ -15,7 +15,9 @@
 #include <asm/e820.h>
 #include "pci.h"
 
-#define MMCONFIG_APER_SIZE (256*1024*1024)
+/* aperture is up to 256MB but BIOS may reserve less */
+#define MMCONFIG_APER_MIN	(2 * 1024*1024)
+#define MMCONFIG_APER_MAX	(256 * 1024*1024)
 
 /* Assume systems with more busses have correct MCFG */
 #define MAX_CHECK_BUS 16
@@ -197,9 +199,10 @@ void __init pci_mmcfg_init(void)
 		return;
 
 	if (!e820_all_mapped(pci_mmcfg_config[0].base_address,
-			pci_mmcfg_config[0].base_address + MMCONFIG_APER_SIZE,
+			pci_mmcfg_config[0].base_address + MMCONFIG_APER_MIN,
 			E820_RESERVED)) {
-		printk(KERN_ERR "PCI: BIOS Bug: MCFG area is not E820-reserved\n");
+		printk(KERN_ERR "PCI: BIOS Bug: MCFG area at %x is not E820-reserved\n",
+				pci_mmcfg_config[0].base_address);
 		printk(KERN_ERR "PCI: Not using MMCONFIG.\n");
 		return;
 	}
diff --git a/arch/x86_64/pci/mmconfig.c b/arch/x86_64/pci/mmconfig.c
index a2060e4d5de6..3c55c76c6fd5 100644
--- a/arch/x86_64/pci/mmconfig.c
+++ b/arch/x86_64/pci/mmconfig.c
@@ -13,7 +13,10 @@
 
 #include "pci.h"
 
-#define MMCONFIG_APER_SIZE (256*1024*1024)
+/* aperture is up to 256MB but BIOS may reserve less */
+#define MMCONFIG_APER_MIN	(2 * 1024*1024)
+#define MMCONFIG_APER_MAX	(256 * 1024*1024)
+
 /* Verify the first 16 busses. We assume that systems with more busses
    get MCFG right. */
 #define MAX_CHECK_BUS 16
@@ -175,9 +178,10 @@ void __init pci_mmcfg_init(void)
 		return;
 
 	if (!e820_all_mapped(pci_mmcfg_config[0].base_address,
-			pci_mmcfg_config[0].base_address + MMCONFIG_APER_SIZE,
+			pci_mmcfg_config[0].base_address + MMCONFIG_APER_MIN,
 			E820_RESERVED)) {
-		printk(KERN_ERR "PCI: BIOS Bug: MCFG area is not E820-reserved\n");
+		printk(KERN_ERR "PCI: BIOS Bug: MCFG area at %x is not E820-reserved\n",
+				pci_mmcfg_config[0].base_address);
 		printk(KERN_ERR "PCI: Not using MMCONFIG.\n");
 		return;
 	}
@@ -190,7 +194,8 @@ void __init pci_mmcfg_init(void)
 	}
 	for (i = 0; i < pci_mmcfg_config_num; ++i) {
 		pci_mmcfg_virt[i].cfg = &pci_mmcfg_config[i];
-		pci_mmcfg_virt[i].virt = ioremap_nocache(pci_mmcfg_config[i].base_address, MMCONFIG_APER_SIZE);
+		pci_mmcfg_virt[i].virt = ioremap_nocache(pci_mmcfg_config[i].base_address,
+							 MMCONFIG_APER_MAX);
 		if (!pci_mmcfg_virt[i].virt) {
 			printk("PCI: Cannot map mmconfig aperture for segment %d\n",
 			       pci_mmcfg_config[i].pci_segment_group_number);