Index: head/sys/cam/ata/ata_xpt.c
===================================================================
--- head/sys/cam/ata/ata_xpt.c
+++ head/sys/cam/ata/ata_xpt.c
@@ -40,6 +40,7 @@
 #include <sys/interrupt.h>
 #include <sys/sbuf.h>
 
+#include <sys/eventhandler.h>
 #include <sys/lock.h>
 #include <sys/mutex.h>
 #include <sys/sysctl.h>
@@ -872,12 +873,24 @@
 	{
 		struct ccb_pathinq cpi;
 		int16_t *ptr;
+		int veto = 0;
 
 		ident_buf = &softc->ident_data;
 		for (ptr = (int16_t *)ident_buf;
 		     ptr < (int16_t *)ident_buf + sizeof(struct ata_params)/2; ptr++) {
 			*ptr = le16toh(*ptr);
 		}
+
+		/*
+		 * Allow others to veto this ATA disk attachment.  This
+		 * is mainly used by VMs, whose disk controllers may
+		 * share the disks with the simulated ATA controllers.
+		 */
+		EVENTHANDLER_INVOKE(ada_probe_veto, path, ident_buf, &veto);
+		if (veto) {
+			goto device_fail;
+		}
+
 		if (strncmp(ident_buf->model, "FX", 2) &&
 		    strncmp(ident_buf->model, "NEC", 3) &&
 		    strncmp(ident_buf->model, "Pioneer", 7) &&
Index: head/sys/sys/eventhandler.h
===================================================================
--- head/sys/sys/eventhandler.h
+++ head/sys/sys/eventhandler.h
@@ -270,4 +270,11 @@
 EVENTHANDLER_DECLARE(register_framebuffer, register_framebuffer_fn);
 EVENTHANDLER_DECLARE(unregister_framebuffer, unregister_framebuffer_fn);
 
+/* Veto ada attachment */
+struct cam_path;
+struct ata_params;
+typedef void (*ada_probe_veto_fn)(void *, struct cam_path *,
+    struct ata_params *, int *);
+EVENTHANDLER_DECLARE(ada_probe_veto, ada_probe_veto_fn);
+
 #endif /* _SYS_EVENTHANDLER_H_ */