Index: sys/cam/cam_iosched.h
===================================================================
--- sys/cam/cam_iosched.h
+++ sys/cam/cam_iosched.h
@@ -41,6 +41,44 @@
 union ccb;
 struct bio;
 
+/*
+ * For 64-bit platforms, we know that uintptr_t is the same size as sbintime_t
+ * so we can store values in it. For 32-bit systems, however, uintptr_t is only
+ * 32-bits, so it won't fit. For those systems, store 24 bits of fraction and 8
+ * bits of seconds. This allows us to measure an interval of up to ~256s, which
+ * is ~200x what our current uses require. Provide some convenience functions to
+ * get the time, subtract two times and convert back to sbintime_t in a safe way
+ * that can be centralized.
+ */
+#ifdef __LP64__
+#define CAM_IOSCHED_TIME_SHIFT 0
+#else
+#define CAM_IOSCHED_TIME_SHIFT 8
+#endif
+static inline uintptr_t
+cam_iosched_now(void)
+{
+
+	/* Cast here is to avoid right shifting a signed value */
+	return (uintptr_t)((uint64_t)sbinuptime() >> CAM_IOSCHED_TIME_SHIFT);
+}
+
+static inline uintptr_t
+cam_iosched_delta_t(uintptr_t then)
+{
+
+	/* Since the types are identical, wrapping works correctly */
+	return (cam_iosched_now() - then);
+}
+
+static inline sbintime_t
+cam_iosched_sbintime_t(uintptr_t delta)
+{
+
+	/* Cast here is to widen the type so the left shift doesn't lose precision */
+	return (sbintime_t)((uint64_t)delta << CAM_IOSCHED_TIME_SHIFT);
+}
+
 int cam_iosched_init(struct cam_iosched_softc **, struct cam_periph *periph);
 void cam_iosched_fini(struct cam_iosched_softc *);
 void cam_iosched_sysctl_init(struct cam_iosched_softc *, struct sysctl_ctx_list *, struct sysctl_oid *);
Index: sys/cam/cam_iosched.c
===================================================================
--- sys/cam/cam_iosched.c
+++ sys/cam/cam_iosched.c
@@ -1428,7 +1428,8 @@
 	}
 
 	if (!(bp->bio_flags & BIO_ERROR))
-		cam_iosched_io_metric_update(isc, done_ccb->ccb_h.qos.sim_data,
+		cam_iosched_io_metric_update(isc,
+		    cam_iosched_sbintime_t(done_ccb->ccb_h.qos.periph_data),
 		    bp->bio_cmd, bp->bio_bcount);
 #endif
 	return retval;
Index: sys/cam/cam_xpt.c
===================================================================
--- sys/cam/cam_xpt.c
+++ sys/cam/cam_xpt.c
@@ -55,6 +55,7 @@
 
 #include <cam/cam.h>
 #include <cam/cam_ccb.h>
+#include <cam/cam_iosched.h>
 #include <cam/cam_periph.h>
 #include <cam/cam_queue.h>
 #include <cam/cam_sim.h>
@@ -3494,7 +3495,7 @@
 			mtx_lock(mtx);
 		else
 			mtx = NULL;
-		work_ccb->ccb_h.qos.sim_data = sbinuptime(); // xxx uintprt_t too small 32bit platforms
+		work_ccb->ccb_h.qos.periph_data = cam_iosched_now();
 		(*(sim->sim_action))(sim, work_ccb);
 		if (mtx)
 			mtx_unlock(mtx);
@@ -4641,7 +4642,7 @@
 		return;
 
 	/* Store the time the ccb was in the sim */
-	done_ccb->ccb_h.qos.sim_data = sbinuptime() - done_ccb->ccb_h.qos.sim_data;
+	done_ccb->ccb_h.qos.periph_data = cam_iosched_delta_t(done_ccb->ccb_h.qos.periph_data);
 	hash = (done_ccb->ccb_h.path_id + done_ccb->ccb_h.target_id +
 	    done_ccb->ccb_h.target_lun) % cam_num_doneqs;
 	queue = &cam_doneqs[hash];
@@ -4664,7 +4665,7 @@
 		return;
 
 	/* Store the time the ccb was in the sim */
-	done_ccb->ccb_h.qos.sim_data = sbinuptime() - done_ccb->ccb_h.qos.sim_data;
+	done_ccb->ccb_h.qos.periph_data = cam_iosched_delta_t(done_ccb->ccb_h.qos.periph_data);
 	xpt_done_process(&done_ccb->ccb_h);
 }