--- src/sys/amd64/amd64/machdep.c	2007-11-15 23:00:57.000000000 +0100
+++ src/sys/amd64/amd64/machdep.c	2007-11-20 09:44:24.000000000 +0100
@@ -155,6 +155,9 @@
 extern vm_offset_t ksym_start, ksym_end;
 #endif
 
+/* Intel ICH register */
+#define ICH_PMBASE     0x400
+
 int	_udatasel, _ucodesel, _ucode32sel;
 
 int cold = 1;
@@ -192,6 +195,27 @@
 cpu_startup(dummy)
 	void *dummy;
 {
+	char *ichenv, *sysenv;
+	
+	/*
+	 * On some systems, namely MacBooks, we need to disallow the
+	 * legacy USB circuit to generate an SMI# because this can
+	 * cause several problems, namely: incorrect CPU frequency
+	 * detection and failure to start the APs.
+	 */
+	ichenv = getenv("hw.ich.disable_legacy_usb");
+	sysenv = getenv("smbios.system.product");
+	if ((ichenv !=  NULL) || (sysenv != NULL &&
+	    strncmp(sysenv, "MacBook", 7) == 0)) {
+		if (bootverbose)
+			printf("Disabling LEGACY_USB_EN bit on Intel ICH.\n");
+		outl(ICH_PMBASE + 0x30, inl(ICH_PMBASE + 0x30) & ~0x8);
+	}
+	if (ichenv)
+		freeenv(ichenv);
+	if (sysenv)
+		freeenv(sysenv);
+
 	/*
 	 * Good {morning,afternoon,evening,night}.
 	 */