Index: if_mcx.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/if_mcx.c,v
retrieving revision 1.71
diff -u -p -r1.71 if_mcx.c
--- if_mcx.c	23 Jul 2020 01:24:53 -0000	1.71
+++ if_mcx.c	27 Nov 2020 06:40:12 -0000
@@ -1478,6 +1478,7 @@ struct mcx_cq_entry {
 	uint32_t		__reserved__[2];
 	uint32_t		cq_byte_cnt;
 	uint64_t		cq_timestamp;
+#define MCX_CQ_ENTRY_TIMESTAMP_MASK		0x7fffffffffffffffULL
 	uint8_t			cq_rx_drops;
 	uint8_t			cq_flow_tag[3];
 	uint16_t		cq_wqe_count;
@@ -6436,7 +6437,9 @@ mcx_process_rx(struct mcx_softc *sc, str
 #endif
 
 	if (c->c_tdiff) {
-		uint64_t t = bemtoh64(&cqe->cq_timestamp) - c->c_timestamp;
+		uint64_t t = bemtoh64(&cqe->cq_timestamp);
+		t &= MCX_CQ_ENTRY_TIMESTAMP_MASK;
+		t -= c->c_timestamp;
 		t *= c->c_udiff;
 		t /= c->c_tdiff;