diff --git a/sys/dev/acpica/Osd/OsdHardware.c b/sys/dev/acpica/Osd/OsdHardware.c
index 0a1bb1a5282..4a612700838 100644
--- a/sys/dev/acpica/Osd/OsdHardware.c
+++ b/sys/dev/acpica/Osd/OsdHardware.c
@@ -34,6 +34,7 @@ __FBSDID("$FreeBSD$");
 
 #include <contrib/dev/acpica/acpi.h>
 
+#include <sys/kernel.h>
 #include <machine/bus.h>
 #include <machine/pci_cfgreg.h>
 #include <dev/pci/pcireg.h>
@@ -62,9 +63,74 @@ __FBSDID("$FreeBSD$");
 #define ACPI_BUS_HANDLE		0
 #endif
 
+/*
+ * Some BIOS vendors use AML to read/write directly to IO space.  This
+ * can cause a problem if such accesses interfere with the OS's access to
+ * the same ports.  Windows XP and newer systems block accesses to certain
+ * IO ports.  We print a message or block accesses based on a tunable.
+ */
+static int illegal_bios_ports[] = {
+	0x000, 0x00f,	/* DMA controller 1 */
+	0x020, 0x021,	/* PIC */
+	0x040, 0x043,	/* Timer 1 */
+	0x048, 0x04b,	/* Timer 2 failsafe */
+	0x070, 0x071,	/* CMOS and RTC */
+	0x074, 0x076,	/* Extended CMOS */
+	0x081, 0x083,	/* DMA1 page registers */
+	0x087, 0x087,	/* DMA1 ch0 low page */
+	0x089, 0x08b,	/* DMA2 ch2 (0x89), ch3 low page (0x8a, 0x8b) */
+	0x08f, 0x091,	/* DMA2 low page refresh (0x8f) */
+			/* Arb ctrl port, card select feedback (0x90, 0x91) */
+	0x093, 0x094,	/* System board setup */
+	0x096, 0x097,	/* POS channel select */
+	0x0a0, 0x0a1,	/* PIC (cascaded) */
+	0x0c0, 0x0df,	/* ISA DMA */
+	0x4d0, 0x4d1,	/* PIC ELCR (edge/level control) */
+	0xcf8, 0xcff,	/* PCI config space. Microsoft adds 0xd00 also but
+			   that seems incorrect. */
+	-1, -1
+};
+
+/* Block accesses to bad IO port addresses or just print a warning. */
+static int block_bad_io;
+TUNABLE_INT("debug.acpi.block_bad_io", &block_bad_io);
+
+/*
+ * Look up bad ports in our table.  Returns 0 if ok, 1 if marked bad but
+ * access is still allowed, or -1 to deny access.
+ */
+static int
+acpi_os_check_port(UINT32 addr, UINT32 width)
+{
+	int error, *port;
+
+	error = 0;
+	for (port = illegal_bios_ports; *port != -1; port += 2) {
+		if ((addr >= port[0] && addr <= port[1]) ||
+		    (addr < port[0] && addr + (width / 8) >= port[0])) {
+			if (block_bad_io)
+			    error = -1;
+			else
+			    error = 1;
+			break;
+		}
+	}
+
+	return (error);
+}
+
 ACPI_STATUS
 AcpiOsReadPort(ACPI_IO_ADDRESS InPort, UINT32 *Value, UINT32 Width)
 {
+    int error;
+
+    error = acpi_os_check_port(InPort, Width);
+    if (error != 0) {
+	printf("acpi: bad read from port 0x%03x (%d)\n", InPort, Width);
+	if (error == -1)
+	    return (AE_BAD_PARAMETER);
+    }
+
     switch (Width) {
     case 8:
         *(u_int8_t *)Value = bus_space_read_1(ACPI_BUS_SPACE_IO,
@@ -89,6 +155,16 @@ AcpiOsReadPort(ACPI_IO_ADDRESS InPort, UINT32 *Value, UINT32 Width)
 ACPI_STATUS
 AcpiOsWritePort(ACPI_IO_ADDRESS OutPort, UINT32	Value, UINT32 Width)
 {
+    int error;
+
+    error = acpi_os_check_port(OutPort, Width);
+    if (error != 0) {
+	printf("acpi: bad write to port 0x%03x (%d), val %#x\n", OutPort,
+	    Width, Value);
+	if (error == -1)
+	    return (AE_BAD_PARAMETER);
+    }
+
     switch (Width) {
     case 8:
         bus_space_write_1(ACPI_BUS_SPACE_IO, ACPI_BUS_HANDLE, OutPort, Value);