diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a626465dd877..faccc582c0f9 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3097,6 +3097,12 @@
 		pcie_scan_all	Scan all possible PCIe devices.  Otherwise we
 				only look for one device below a PCIe downstream
 				port.
+		big_root_window	Try to add a big 64bit memory window to the PCIe
+				root complex on AMD CPUs. Some GFX hardware
+				can resize a BAR to allow access to all VRAM.
+				Adding the window is slightly risky (it may
+				conflict with unreported devices), so this
+				taints the kernel.
 
 	pcie_aspm=	[PCIE] Forcibly enable or disable PCIe Active State Power
 			Management.
diff --git a/arch/x86/include/asm/pci_x86.h b/arch/x86/include/asm/pci_x86.h
index 7a5d6695abd3..eb66fa9cd0fc 100644
--- a/arch/x86/include/asm/pci_x86.h
+++ b/arch/x86/include/asm/pci_x86.h
@@ -38,6 +38,7 @@ do {						\
 #define PCI_NOASSIGN_ROMS	0x80000
 #define PCI_ROOT_NO_CRS		0x100000
 #define PCI_NOASSIGN_BARS	0x200000
+#define PCI_BIG_ROOT_WINDOW	0x400000
 
 extern unsigned int pci_probe;
 extern unsigned long pirq_table_addr;
diff --git a/arch/x86/pci/common.c b/arch/x86/pci/common.c
index 7a5350d08cef..563049c483a1 100644
--- a/arch/x86/pci/common.c
+++ b/arch/x86/pci/common.c
@@ -594,6 +594,11 @@ char *__init pcibios_setup(char *str)
 	} else if (!strcmp(str, "nocrs")) {
 		pci_probe |= PCI_ROOT_NO_CRS;
 		return NULL;
+#ifdef CONFIG_PHYS_ADDR_T_64BIT
+	} else if (!strcmp(str, "big_root_window")) {
+		pci_probe |= PCI_BIG_ROOT_WINDOW;
+		return NULL;
+#endif
 	} else if (!strcmp(str, "earlydump")) {
 		pci_early_dump_regs = 1;
 		return NULL;
diff --git a/arch/x86/pci/fixup.c b/arch/x86/pci/fixup.c
index e663d6bf1328..f6a26e3cb476 100644
--- a/arch/x86/pci/fixup.c
+++ b/arch/x86/pci/fixup.c
@@ -662,10 +662,14 @@ DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_INTEL, 0x2033, quirk_no_aersid);
  */
 static void pci_amd_enable_64bit_bar(struct pci_dev *dev)
 {
-	unsigned i;
 	u32 base, limit, high;
-	struct resource *res, *conflict;
 	struct pci_dev *other;
+	struct resource *res;
+	unsigned i;
+	int r;
+
+	if (!(pci_probe & PCI_BIG_ROOT_WINDOW))
+		return;
 
 	/* Check that we are the only device of that type */
 	other = pci_get_device(dev->vendor, dev->device, NULL);
@@ -699,22 +703,25 @@ static void pci_amd_enable_64bit_bar(struct pci_dev *dev)
 	if (!res)
 		return;
 
+	/*
+	 * Allocate a 256GB window directly below the 0xfd00000000 hardware
+	 * limit (see AMD Family 15h Models 30h-3Fh BKDG, sec 2.4.6).
+	 */
 	res->name = "PCI Bus 0000:00";
 	res->flags = IORESOURCE_PREFETCH | IORESOURCE_MEM |
 		IORESOURCE_MEM_64 | IORESOURCE_WINDOW;
-	res->start = 0x100000000ull;
+	res->start = 0xbd00000000ull;
 	res->end = 0xfd00000000ull - 1;
 
-	/* Just grab the free area behind system memory for this */
-	while ((conflict = request_resource_conflict(&iomem_resource, res))) {
-		if (conflict->end >= res->end) {
-			kfree(res);
-			return;
-		}
-		res->start = conflict->end + 1;
+	r = request_resource(&iomem_resource, res);
+	if (r) {
+		kfree(res);
+		return;
 	}
 
-	dev_info(&dev->dev, "adding root bus resource %pR\n", res);
+	dev_info(&dev->dev, "adding root bus resource %pR (tainting kernel)\n",
+		 res);
+	add_taint(TAINT_FIRMWARE_WORKAROUND, LOCKDEP_STILL_OK);
 
 	base = ((res->start >> 8) & AMD_141b_MMIO_BASE_MMIOBASE_MASK) |
 		AMD_141b_MMIO_BASE_RE_MASK | AMD_141b_MMIO_BASE_WE_MASK;