x2APIC is currently enabled on HV guests only, not on bare metal, to

speedup interrupt handling on virtual machines.  Unfortunately it is
broken on Xen 4.2 or older, so use the early pvbus detection to drop
the advertised x2APIC CPU feature.  Xen has fixed this in late 2013,
but the problem still shows up in "the Cloud" (it doesn't seem to
affect others as they either use legacy APIC or Xen IPIs).

OK mlarkin@ mikeb@
Cvs: ----------------------------------------------------------------------

Author: reyk

sys/dev/pv/pvbus.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: pvbus.c,v 1.9 2015/12/12 12:33:49 reyk Exp $	*/
+/*	$OpenBSD: pvbus.c,v 1.10 2015/12/12 12:47:49 reyk Exp $	*/
 
 /*
  * Copyright (c) 2015 Reyk Floeter <reyk@openbsd.org>
@@ -262,6 +262,13 @@ pvbus_xen(struct pvbus_hv *hv)
 	    regs[0], regs[1], regs[2], regs[3]);
 	hv->hv_major = regs[0] >> XEN_VERSION_MAJOR_S;
 	hv->hv_minor = regs[0] & XEN_VERSION_MINOR_M;
+
+	/* x2apic is broken in Xen 4.2 or older */
+	if ((hv->hv_major < 4) ||
+	    (hv->hv_major == 4 && hv->hv_minor < 3)) {
+		/* Remove CPU flag for x2apic */
+		cpu_ecxfeature &= ~CPUIDECX_X2APIC;
+	}
 }
 
 void