Index: include/cpufunc.h
===================================================================
RCS file: /cvs/src/sys/arch/amd64/include/cpufunc.h,v
retrieving revision 1.36
diff -u -p -r1.36 cpufunc.h
--- include/cpufunc.h	13 Sep 2020 11:53:16 -0000	1.36
+++ include/cpufunc.h	26 Oct 2020 09:19:14 -0000
@@ -293,6 +293,12 @@ mfence(void)
 	__asm volatile("mfence" : : : "memory");
 }
 
+static __inline void
+lfence(void)
+{
+	__asm volatile("lfence" : : : "memory");
+}
+
 static __inline u_int64_t
 rdtsc(void)
 {
Index: pci/pci_machdep.c
===================================================================
RCS file: /cvs/src/sys/arch/amd64/pci/pci_machdep.c,v
retrieving revision 1.75
diff -u -p -r1.75 pci_machdep.c
--- pci/pci_machdep.c	17 Jun 2020 06:14:52 -0000	1.75
+++ pci/pci_machdep.c	26 Oct 2020 09:19:14 -0000
@@ -75,6 +75,7 @@
 #include <machine/pio.h>
 #include <machine/intr.h>
 #include <machine/biosvar.h>
+#include <machine/cpufunc.h>
 
 #include <dev/isa/isareg.h>
 #include <dev/pci/pcivar.h>
@@ -118,6 +119,9 @@ do {									\
 #define	PCI_MODE1_ADDRESS_REG	0x0cf8
 #define	PCI_MODE1_DATA_REG	0x0cfc
 
+void	_pci_bus_dmamap_sync(bus_dma_tag_t, bus_dmamap_t,
+	    bus_addr_t, bus_size_t, int);
+
 /*
  * PCI doesn't have any special needs; just use the generic versions
  * of these functions.
@@ -131,7 +135,7 @@ struct bus_dma_tag pci_bus_dma_tag = {
 	_bus_dmamap_load_uio,
 	_bus_dmamap_load_raw,
 	_bus_dmamap_unload,
-	_bus_dmamap_sync,
+	_pci_bus_dmamap_sync,
 	_bus_dmamem_alloc,
 	_bus_dmamem_alloc_range,
 	_bus_dmamem_free,
@@ -139,6 +143,19 @@ struct bus_dma_tag pci_bus_dma_tag = {
 	_bus_dmamem_unmap,
 	_bus_dmamem_mmap,
 };
+
+void
+_pci_bus_dmamap_sync(bus_dma_tag_t t, bus_dmamap_t map, bus_addr_t offset,
+    bus_size_t len, int op)
+{
+	if (len == 0)
+		return;
+
+	if (ISSET(op, BUS_DMASYNC_PREWRITE|BUS_DMASYNC_POSTWRITE))
+		mfence();
+	else if (ISSET(op, BUS_DMASYNC_PREREAD|BUS_DMASYNC_POSTREAD))
+		lfence();
+}
 
 void
 pci_mcfg_init(bus_space_tag_t iot, bus_addr_t addr, int segment,