Index: sys/amd64/amd64/pmap.c
===================================================================
--- sys/amd64/amd64/pmap.c
+++ sys/amd64/amd64/pmap.c
@@ -639,6 +639,13 @@
 #define	PMAP_ENTER_NORECLAIM	0x1000000	/* Don't reclaim PV entries. */
 #define	PMAP_ENTER_NOREPLACE	0x2000000	/* Don't replace mappings. */
 
+/*
+ * Internal flags for pmap_mapdev_internal() and
+ * pmap_change_attr_locked().
+ */
+#define	MAPDEV_FLUSHCACHE	0x0000001	/* Flush cache after mapping. */
+#define	MAPDEV_SETATTR		0x0000002	/* Modify existing attrs. */
+
 static void	free_pv_chunk(struct pv_chunk *pc);
 static void	free_pv_entry(pmap_t pmap, pv_entry_t pv);
 static pv_entry_t get_pv_entry(pmap_t pmap, struct rwlock **lockp);
@@ -659,7 +666,7 @@
 		    vm_offset_t va);
 
 static int pmap_change_attr_locked(vm_offset_t va, vm_size_t size, int mode,
-    bool noflush);
+    int flags);
 static boolean_t pmap_demote_pde(pmap_t pmap, pd_entry_t *pde, vm_offset_t va);
 static boolean_t pmap_demote_pde_locked(pmap_t pmap, pd_entry_t *pde,
     vm_offset_t va, struct rwlock **lockp);
@@ -7181,7 +7188,7 @@
  * NOT real memory.
  */
 static void *
-pmap_mapdev_internal(vm_paddr_t pa, vm_size_t size, int mode, bool noflush)
+pmap_mapdev_internal(vm_paddr_t pa, vm_size_t size, int mode, int flags)
 {
 	struct pmap_preinit_mapping *ppim;
 	vm_offset_t va, offset;
@@ -7215,7 +7222,7 @@
 		for (i = 0; i < PMAP_PREINIT_MAPPING_COUNT; i++) {
 			ppim = pmap_preinit_mapping + i;
 			if (ppim->pa == pa && ppim->sz == size &&
-			    ppim->mode == mode)
+			    (ppim->mode == mode || !(flags & MAPDEV_SETATTR)))
 				return ((void *)(ppim->va + offset));
 		}
 		/*
@@ -7224,9 +7231,12 @@
 		 */
 		if (pa < dmaplimit && pa + size <= dmaplimit) {
 			va = PHYS_TO_DMAP(pa);
-			PMAP_LOCK(kernel_pmap);
-			i = pmap_change_attr_locked(va, size, mode, noflush);
-			PMAP_UNLOCK(kernel_pmap);
+			if (flags & MAPDEV_SETATTR) {
+				PMAP_LOCK(kernel_pmap);
+				i = pmap_change_attr_locked(va, size, mode, flags);
+				PMAP_UNLOCK(kernel_pmap);
+			} else
+				i = 0;
 			if (!i)
 				return ((void *)(va + offset));
 		}
@@ -7237,7 +7247,7 @@
 	for (tmpsize = 0; tmpsize < size; tmpsize += PAGE_SIZE)
 		pmap_kenter_attr(va + tmpsize, pa + tmpsize, mode);
 	pmap_invalidate_range(kernel_pmap, va, va + tmpsize);
-	if (!noflush)
+	if (flags & MAPDEV_FLUSHCACHE)
 		pmap_invalidate_cache_range(va, va + tmpsize);
 	return ((void *)(va + offset));
 }
@@ -7246,28 +7256,31 @@
 pmap_mapdev_attr(vm_paddr_t pa, vm_size_t size, int mode)
 {
 
-	return (pmap_mapdev_internal(pa, size, mode, false));
+	return (pmap_mapdev_internal(pa, size, mode, MAPDEV_FLUSHCACHE |
+	    MAPDEV_SETATTR));
 }
 
 void *
 pmap_mapdev(vm_paddr_t pa, vm_size_t size)
 {
 
-	return (pmap_mapdev_internal(pa, size, PAT_UNCACHEABLE, false));
+	return (pmap_mapdev_attr(pa, size, PAT_UNCACHEABLE));
 }
 
 void *
 pmap_mapdev_pciecfg(vm_paddr_t pa, vm_size_t size)
 {
 
-	return (pmap_mapdev_internal(pa, size, PAT_UNCACHEABLE, true));
+	return (pmap_mapdev_internal(pa, size, PAT_UNCACHEABLE,
+	    MAPDEV_SETATTR));
 }
 
 void *
 pmap_mapbios(vm_paddr_t pa, vm_size_t size)
 {
 
-	return (pmap_mapdev_internal(pa, size, PAT_WRITE_BACK, false));
+	return (pmap_mapdev_internal(pa, size, PAT_WRITE_BACK,
+	    MAPDEV_FLUSHCACHE));
 }
 
 void
@@ -7406,13 +7419,13 @@
 	int error;
 
 	PMAP_LOCK(kernel_pmap);
-	error = pmap_change_attr_locked(va, size, mode, false);
+	error = pmap_change_attr_locked(va, size, mode, MAPDEV_FLUSHCACHE);
 	PMAP_UNLOCK(kernel_pmap);
 	return (error);
 }
 
 static int
-pmap_change_attr_locked(vm_offset_t va, vm_size_t size, int mode, bool noflush)
+pmap_change_attr_locked(vm_offset_t va, vm_size_t size, int mode, int flags)
 {
 	vm_offset_t base, offset, tmpva;
 	vm_paddr_t pa_start, pa_end, pa_end1;
@@ -7529,7 +7542,7 @@
 					/* Run ended, update direct map. */
 					error = pmap_change_attr_locked(
 					    PHYS_TO_DMAP(pa_start),
-					    pa_end - pa_start, mode, noflush);
+					    pa_end - pa_start, mode, flags);
 					if (error != 0)
 						break;
 					/* Start physical address run. */
@@ -7559,7 +7572,7 @@
 					/* Run ended, update direct map. */
 					error = pmap_change_attr_locked(
 					    PHYS_TO_DMAP(pa_start),
-					    pa_end - pa_start, mode, noflush);
+					    pa_end - pa_start, mode, flags);
 					if (error != 0)
 						break;
 					/* Start physical address run. */
@@ -7587,7 +7600,7 @@
 					/* Run ended, update direct map. */
 					error = pmap_change_attr_locked(
 					    PHYS_TO_DMAP(pa_start),
-					    pa_end - pa_start, mode, noflush);
+					    pa_end - pa_start, mode, flags);
 					if (error != 0)
 						break;
 					/* Start physical address run. */
@@ -7602,7 +7615,7 @@
 		pa_end1 = MIN(pa_end, dmaplimit);
 		if (pa_start != pa_end1)
 			error = pmap_change_attr_locked(PHYS_TO_DMAP(pa_start),
-			    pa_end1 - pa_start, mode, noflush);
+			    pa_end1 - pa_start, mode, flags);
 	}
 
 	/*
@@ -7611,7 +7624,7 @@
 	 */
 	if (changed) {
 		pmap_invalidate_range(kernel_pmap, base, tmpva);
-		if (!noflush)
+		if (flags & MAPDEV_FLUSHCACHE)
 			pmap_invalidate_cache_range(base, tmpva);
 	}
 	return (error);
Index: sys/i386/i386/pmap.c
===================================================================
--- sys/i386/i386/pmap.c
+++ sys/i386/i386/pmap.c
@@ -297,6 +297,9 @@
 #define	PMAP_ENTER_NORECLAIM	0x1000000	/* Don't reclaim PV entries. */
 #define	PMAP_ENTER_NOREPLACE	0x2000000	/* Don't replace mappings. */
 
+/* Internal flags for pmap_mapdev_attr(). */
+#define	MAPDEV_SETATTR		0x0000002	/* Modify existing attrs. */
+
 static void	free_pv_chunk(struct pv_chunk *pc);
 static void	free_pv_entry(pmap_t pmap, pv_entry_t pv);
 static pv_entry_t get_pv_entry(pmap_t pmap, boolean_t try);
@@ -5393,10 +5396,12 @@
  * NOT real memory.
  */
 static void *
-__CONCAT(PMTYPE, mapdev_attr)(vm_paddr_t pa, vm_size_t size, int mode)
+__CONCAT(PMTYPE, mapdev_attr)(vm_paddr_t pa, vm_size_t size, int mode,
+    int flags)
 {
 	struct pmap_preinit_mapping *ppim;
 	vm_offset_t va, offset;
+	vm_page_t m;
 	vm_size_t tmpsize;
 	int i;
 
@@ -5404,9 +5409,11 @@
 	size = round_page(offset + size);
 	pa = pa & PG_FRAME;
 
-	if (pa < PMAP_MAP_LOW && pa + size <= PMAP_MAP_LOW)
+	if (pa < PMAP_MAP_LOW && pa + size <= PMAP_MAP_LOW) {
 		va = pa + PMAP_MAP_LOW;
-	else if (!pmap_initialized) {
+		if (!(flags & MAPDEV_SETATTR))
+			return ((void *)(va + offset));
+	} else if (!pmap_initialized) {
 		va = 0;
 		for (i = 0; i < PMAP_PREINIT_MAPPING_COUNT; i++) {
 			ppim = pmap_preinit_mapping + i;
@@ -5429,15 +5436,24 @@
 		for (i = 0; i < PMAP_PREINIT_MAPPING_COUNT; i++) {
 			ppim = pmap_preinit_mapping + i;
 			if (ppim->pa == pa && ppim->sz == size &&
-			    ppim->mode == mode)
+			    (ppim->mode == mode || !(flags & MAPDEV_SETATTR)))
 				return ((void *)(ppim->va + offset));
 		}
 		va = kva_alloc(size);
 		if (va == 0)
 			panic("%s: Couldn't allocate KVA", __func__);
 	}
-	for (tmpsize = 0; tmpsize < size; tmpsize += PAGE_SIZE)
+	for (tmpsize = 0; tmpsize < size; tmpsize += PAGE_SIZE) {
+		if (!(flags & MAPDEV_SETATTR) && pmap_initialized) {
+			m = PHYS_TO_VM_PAGE(pa);
+			if (m != NULL) {
+				pmap_kenter_attr(va + tmpsize, pa + tmpsize,
+				    m->md.pat_mode);
+				continue;
+			}
+		}
 		pmap_kenter_attr(va + tmpsize, pa + tmpsize, mode);
+	}
 	pmap_invalidate_range_int(kernel_pmap, va, va + tmpsize);
 	pmap_invalidate_cache_range(va, va + size);
 	return ((void *)(va + offset));
Index: sys/i386/i386/pmap_base.c
===================================================================
--- sys/i386/i386/pmap_base.c
+++ sys/i386/i386/pmap_base.c
@@ -777,21 +777,23 @@
 pmap_mapdev_attr(vm_paddr_t pa, vm_size_t size, int mode)
 {
 
-	return (pmap_methods_ptr->pm_mapdev_attr(pa, size, mode));
+	return (pmap_methods_ptr->pm_mapdev_attr(pa, size, mode,
+	    MAPDEV_SETATTR));
 }
 
 void *
 pmap_mapdev(vm_paddr_t pa, vm_size_t size)
 {
 
-	return (pmap_methods_ptr->pm_mapdev_attr(pa, size, PAT_UNCACHEABLE));
+	return (pmap_methods_ptr->pm_mapdev_attr(pa, size, PAT_UNCACHEABLE,
+	    MAPDEV_SETATTR));
 }
 
 void *
 pmap_mapbios(vm_paddr_t pa, vm_size_t size)
 {
 
-	return (pmap_methods_ptr->pm_mapdev_attr(pa, size, PAT_WRITE_BACK));
+	return (pmap_methods_ptr->pm_mapdev_attr(pa, size, PAT_WRITE_BACK, 0));
 }
 
 void
Index: sys/i386/include/pmap_base.h
===================================================================
--- sys/i386/include/pmap_base.h
+++ sys/i386/include/pmap_base.h
@@ -34,6 +34,9 @@
 #ifndef _MACHINE_PMAP_BASE_H_
 #define	_MACHINE_PMAP_BASE_H_
 
+/* Internal flags for pmap_mapdev_attr(). */
+#define	MAPDEV_SETATTR		0x0000002	/* Modify existing attrs. */
+
 struct pmap_methods {
 	void (*pm_ksetrw)(vm_offset_t);
 	void (*pm_remap_lower)(bool);
@@ -93,7 +96,7 @@
 	boolean_t (*pm_is_referenced)(vm_page_t);
 	void (*pm_remove_write)(vm_page_t);
 	int (*pm_ts_referenced)(vm_page_t);
-	void *(*pm_mapdev_attr)(vm_paddr_t, vm_size_t, int);
+	void *(*pm_mapdev_attr)(vm_paddr_t, vm_size_t, int, int);
 	void (*pm_unmapdev)(vm_offset_t, vm_size_t);
 	void (*pm_page_set_memattr)(vm_page_t, vm_memattr_t);
 	vm_paddr_t (*pm_extract)(pmap_t, vm_offset_t);