diff --git a/sys/kern/subr_physmem.c b/sys/kern/subr_physmem.c
index 1fb617078625..b76d25ed1ae2 100644
--- a/sys/kern/subr_physmem.c
+++ b/sys/kern/subr_physmem.c
@@ -1,638 +1,651 @@
 /*-
  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
  *
  * Copyright (c) 2014 Ian Lepore <ian@freebsd.org>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
 
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
 #ifdef _KERNEL
 #include "opt_acpi.h"
 #include "opt_ddb.h"
 #endif
 
 /*
  * Routines for describing and initializing anything related to physical memory.
  */
 
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/bus.h>
 #include <sys/kernel.h>
 #include <sys/module.h>
 #include <sys/physmem.h>
 
 #ifdef _KERNEL
 #include <vm/vm.h>
 #include <vm/vm_param.h>
 #include <vm/vm_page.h>
 #include <vm/vm_phys.h>
 #include <vm/vm_dumpset.h>
 
 #include <machine/md_var.h>
 #include <machine/resource.h>
 #else
 #include <stdarg.h>
 #include <stdio.h>
 #include <string.h>
 #endif
 
 /*
  * These structures are used internally to keep track of regions of physical
  * ram, and regions within the physical ram that need to be excluded.  An
  * exclusion region can be excluded from crash dumps, from the vm pool of pages
  * that can be allocated, or both, depending on the exclusion flags associated
  * with the region.
  */
 #ifdef DEV_ACPI
 #define	MAX_HWCNT	32	/* ACPI needs more regions */
 #define	MAX_EXCNT	32
 #else
 #define	MAX_HWCNT	16
 #define	MAX_EXCNT	16
 #endif
 
 #if defined(__arm__)
 #define	MAX_PHYS_ADDR	0xFFFFFFFFull
 #elif defined(__aarch64__) || defined(__amd64__) || defined(__riscv)
 #define	MAX_PHYS_ADDR	0xFFFFFFFFFFFFFFFFull
 #endif
 
 struct region {
 	vm_paddr_t	addr;
 	vm_size_t	size;
 	uint32_t	flags;
 };
 
 static struct region hwregions[MAX_HWCNT];
 static struct region exregions[MAX_EXCNT];
 
 static size_t hwcnt;
 static size_t excnt;
 
 /*
  * realmem is the total number of hardware pages, excluded or not.
  * Maxmem is one greater than the last physical page number.
  */
 long realmem;
 long Maxmem;
 
 #ifndef _KERNEL
 static void
 panic(const char *fmt, ...)
 {
 	va_list va;
 
 	va_start(va, fmt);
 	vfprintf(stderr, fmt, va);
 	fprintf(stderr, "\n");
 	va_end(va);
 	__builtin_trap();
 }
 #endif
 
 /*
  * Print the contents of the physical and excluded region tables using the
  * provided printf-like output function (which will be either printf or
  * db_printf).
  */
 static void
 physmem_dump_tables(int (*prfunc)(const char *, ...) __printflike(1, 2))
 {
 	size_t i;
 	int flags;
 	uintmax_t addr, size;
 	const unsigned int mbyte = 1024 * 1024;
 
 	prfunc("Physical memory chunk(s):\n");
 	for (i = 0; i < hwcnt; ++i) {
 		addr = hwregions[i].addr;
 		size = hwregions[i].size;
 		prfunc("  0x%08jx - 0x%08jx, %5ju MB (%7ju pages)\n", addr,
 		    addr + size - 1, size / mbyte, size / PAGE_SIZE);
 	}
 
 	prfunc("Excluded memory regions:\n");
 	for (i = 0; i < excnt; ++i) {
 		addr  = exregions[i].addr;
 		size  = exregions[i].size;
 		flags = exregions[i].flags;
 		prfunc("  0x%08jx - 0x%08jx, %5ju MB (%7ju pages) %s %s\n",
 		    addr, addr + size - 1, size / mbyte, size / PAGE_SIZE,
 		    (flags & EXFLAG_NOALLOC) ? "NoAlloc" : "",
 		    (flags & EXFLAG_NODUMP)  ? "NoDump" : "");
 	}
 
 #ifdef DEBUG
 	prfunc("Avail lists:\n");
 	for (i = 0; phys_avail[i] != 0; ++i) {
 		prfunc("  phys_avail[%zu] 0x%08jx\n", i,
 		    (uintmax_t)phys_avail[i]);
 	}
 	for (i = 0; dump_avail[i] != 0; ++i) {
 		prfunc("  dump_avail[%zu] 0x%08jx\n", i,
 		    (uintmax_t)dump_avail[i]);
 	}
 #endif
 }
 
 /*
  * Print the contents of the static mapping table.  Used for bootverbose.
  */
 void
 physmem_print_tables(void)
 {
 
 	physmem_dump_tables(printf);
 }
 
 /*
  * Walk the list of hardware regions, processing it against the list of
  * exclusions that contain the given exflags, and generating an "avail list".
  * 
  * If maxphyssz is not zero it sets upper limit, in bytes, for the total
  * "avail list" size. Walk stops once the limit is reached and the last region
  * is cut short if necessary.
  *
  * Updates the value at *pavail with the sum of all pages in all hw regions.
  *
  * Returns the number of pages of non-excluded memory added to the avail list.
  */
 static size_t
 regions_to_avail(vm_paddr_t *avail, uint32_t exflags, size_t maxavail,
     uint64_t maxphyssz, long *pavail, long *prealmem)
 {
 	size_t acnt, exi, hwi;
 	uint64_t adj, end, start, xend, xstart;
 	long availmem, totalmem;
 	const struct region *exp, *hwp;
 	uint64_t availsz;
 
 	totalmem = 0;
 	availmem = 0;
 	availsz = 0;
 	acnt = 0;
 	for (hwi = 0, hwp = hwregions; hwi < hwcnt; ++hwi, ++hwp) {
 		adj   = round_page(hwp->addr) - hwp->addr;
 		start = round_page(hwp->addr);
 		end   = trunc_page(hwp->size + adj) + start;
 		totalmem += atop((vm_offset_t)(end - start));
 		for (exi = 0, exp = exregions; exi < excnt; ++exi, ++exp) {
 			/*
 			 * If the excluded region does not match given flags,
 			 * continue checking with the next excluded region.
 			 */
 			if ((exp->flags & exflags) == 0)
 				continue;
 			xstart = exp->addr;
 			xend   = exp->size + xstart;
 			/*
 			 * If the excluded region ends before this hw region,
 			 * continue checking with the next excluded region.
 			 */
 			if (xend <= start)
 				continue;
 			/*
 			 * If the excluded region begins after this hw region
 			 * we're done because both lists are sorted.
 			 */
 			if (xstart >= end)
 				break;
 			/*
 			 * If the excluded region completely covers this hw
 			 * region, shrink this hw region to zero size.
 			 */
 			if ((start >= xstart) && (end <= xend)) {
 				start = xend;
 				end = xend;
 				break;
 			}
 			/*
 			 * If the excluded region falls wholly within this hw
 			 * region without abutting or overlapping the beginning
 			 * or end, create an available entry from the leading
 			 * fragment, then adjust the start of this hw region to
 			 * the end of the excluded region, and continue checking
 			 * the next excluded region because another exclusion
 			 * could affect the remainder of this hw region.
 			 */
 			if ((xstart > start) && (xend < end)) {
 
 				if ((maxphyssz != 0) &&
 				    (availsz + xstart - start > maxphyssz)) {
 					xstart = maxphyssz + start - availsz;
 				}
 				if (xstart <= start)
 					continue;
 				if (acnt > 0 &&
 				    avail[acnt - 1] == (vm_paddr_t)start) {
 					avail[acnt - 1] = (vm_paddr_t)xstart;
 				} else {
 					avail[acnt++] = (vm_paddr_t)start;
 					avail[acnt++] = (vm_paddr_t)xstart;
 				}
 				availsz += (xstart - start);
 				availmem += atop((vm_offset_t)(xstart - start));
 				start = xend;
 				continue;
 			}
 			/*
 			 * We know the excluded region overlaps either the start
 			 * or end of this hardware region (but not both), trim
 			 * the excluded portion off the appropriate end.
 			 */
 			if (xstart <= start)
 				start = xend;
 			else
 				end = xstart;
 		}
 		/*
 		 * If the trimming actions above left a non-zero size, create an
 		 * available entry for it.
 		 */
 		if (end > start) {
 			if ((maxphyssz != 0) &&
 			    (availsz + end - start > maxphyssz)) {
 				end = maxphyssz + start - availsz;
 			}
 			if (end <= start)
 				break;
 
 			if (acnt > 0 && avail[acnt - 1] == (vm_paddr_t)start) {
 				avail[acnt - 1] = (vm_paddr_t)end;
 			} else {
 				avail[acnt++] = (vm_paddr_t)start;
 				avail[acnt++] = (vm_paddr_t)end;
 			}
 			availsz += end - start;
 			availmem += atop((vm_offset_t)(end - start));
 		}
 		if (acnt >= maxavail)
 			panic("Not enough space in the dump/phys_avail arrays");
 	}
 
 	if (pavail != NULL)
 		*pavail = availmem;
 	if (prealmem != NULL)
 		*prealmem = totalmem;
 	return (acnt);
 }
 
 /*
  * Check if the region at idx can be merged with the region above it.
  */
 static size_t
 merge_upper_regions(struct region *regions, size_t rcnt, size_t idx)
 {
 	struct region *lower, *upper;
 	vm_paddr_t lend, uend;
 	size_t i, mergecnt, movecnt;
 
 	lower = &regions[idx];
 	lend = lower->addr + lower->size;
 
 	/*
 	 * Continue merging in upper entries as long as we have entries to
 	 * merge; the new block could have spanned more than one, although one
 	 * is likely the common case.
 	 */
 	for (i = idx + 1; i < rcnt; i++) {
 		upper = &regions[i];
 		if (lend < upper->addr || lower->flags != upper->flags)
 			break;
 
 		uend = upper->addr + upper->size;
 		if (uend > lend) {
 			lower->size += uend - lend;
 			lend = lower->addr + lower->size;
 		}
 
 		if (uend >= lend) {
 			/*
 			 * If we didn't move past the end of the upper region,
 			 * then we don't need to bother checking for another
 			 * merge because it would have been done already.  Just
 			 * increment i once more to maintain the invariant that
 			 * i is one past the last entry merged.
 			 */
 			i++;
 			break;
 		}
 	}
 
 	/*
 	 * We merged in the entries from [idx + 1, i); physically move the tail
 	 * end at [i, rcnt) if we need to.
 	 */
 	mergecnt = i - (idx + 1);
 	if (mergecnt > 0) {
 		movecnt = rcnt - i;
 		if (movecnt == 0) {
 			/* Merged all the way to the end, just decrease rcnt. */
 			rcnt = idx + 1;
 		} else {
 			memmove(&regions[idx + 1], &regions[idx + mergecnt + 1],
 			    movecnt * sizeof(*regions));
 			rcnt -= mergecnt;
 		}
 	}
 	return (rcnt);
 }
 
 /*
  * Insertion-sort a new entry into a regions list; sorted by start address.
  */
 static size_t
 insert_region(struct region *regions, size_t rcnt, vm_paddr_t addr,
     vm_size_t size, uint32_t flags)
 {
 	size_t i;
 	vm_paddr_t nend, rend;
 	struct region *ep, *rp;
 
 	nend = addr + size;
 	ep = regions + rcnt;
 	for (i = 0, rp = regions; i < rcnt; ++i, ++rp) {
+		rend = rp->addr + rp->size;
 		if (flags == rp->flags) {
-			rend = rp->addr + rp->size;
 			if (addr <= rp->addr && nend >= rp->addr) {
 				/*
 				 * New mapping overlaps at the beginning, shift
 				 * for any difference in the beginning then
 				 * shift if the new mapping extends past.
 				 */
 				rp->size += rp->addr - addr;
 				rp->addr = addr;
 				if (nend > rend) {
 					rp->size += nend - rend;
 					rcnt = merge_upper_regions(regions,
 					    rcnt, i);
 				}
 				return (rcnt);
 			} else if (addr <= rend && nend > rp->addr) {
 				/*
 				 * New mapping is either entirely contained
 				 * within or it's overlapping at the end.
 				 */
 				if (nend > rend) {
 					rp->size += nend - rend;
 					rcnt = merge_upper_regions(regions,
 					    rcnt, i);
 				}
 				return (rcnt);
 			}
+		} else if ((flags != 0) && (rp->flags != 0)) {
+			/*
+			 * If we're duplicating an entry that already exists
+			 * exactly, just upgrade its flags as needed.  We could
+			 * do more if we find that we have differently specified
+			 * flags clipping existing excluding regions, but that's
+			 * probably rare.
+			 */
+			if (addr == rp->addr && nend == rend) {
+				rp->flags |= flags;
+				return (rcnt);
+			}
 		}
+
 		if (addr < rp->addr) {
 			bcopy(rp, rp + 1, (ep - rp) * sizeof(*rp));
 			break;
 		}
 	}
 	rp->addr  = addr;
 	rp->size  = size;
 	rp->flags = flags;
 	rcnt++;
 
 	return (rcnt);
 }
 
 /*
  * Add a hardware memory region.
  */
 void
 physmem_hardware_region(uint64_t pa, uint64_t sz)
 {
 	/*
 	 * Filter out the page at PA 0x00000000.  The VM can't handle it, as
 	 * pmap_extract() == 0 means failure.
 	 */
 	if (pa == 0) {
 		if (sz <= PAGE_SIZE)
 			return;
 		pa  = PAGE_SIZE;
 		sz -= PAGE_SIZE;
 	} else if (pa > MAX_PHYS_ADDR) {
 		/* This range is past usable memory, ignore it */
 		return;
 	}
 
 	/*
 	 * Also filter out the page at the end of the physical address space --
 	 * if addr is non-zero and addr+size is zero we wrapped to the next byte
 	 * beyond what vm_paddr_t can express.  That leads to a NULL pointer
 	 * deref early in startup; work around it by leaving the last page out.
 	 *
 	 * XXX This just in:  subtract out a whole megabyte, not just 1 page.
 	 * Reducing the size by anything less than 1MB results in the NULL
 	 * pointer deref in _vm_map_lock_read().  Better to give up a megabyte
 	 * than leave some folks with an unusable system while we investigate.
 	 */
 	if ((pa + sz) > (MAX_PHYS_ADDR - 1024 * 1024)) {
 		sz = MAX_PHYS_ADDR - pa + 1;
 		if (sz <= 1024 * 1024)
 			return;
 		sz -= 1024 * 1024;
 	}
 
 	if (sz > 0 && hwcnt < nitems(hwregions))
 		hwcnt = insert_region(hwregions, hwcnt, pa, sz, 0);
 }
 
 /*
  * Add an exclusion region.
  */
 void
 physmem_exclude_region(vm_paddr_t pa, vm_size_t sz, uint32_t exflags)
 {
 	vm_offset_t adj;
 
 	/*
 	 * Truncate the starting address down to a page boundary, and round the
 	 * ending page up to a page boundary.
 	 */
 	adj = pa - trunc_page(pa);
 	pa  = trunc_page(pa);
 	sz  = round_page(sz + adj);
 
 	if (excnt >= nitems(exregions))
 		panic("failed to exclude region %#jx-%#jx", (uintmax_t)pa,
 		    (uintmax_t)(pa + sz));
 	excnt = insert_region(exregions, excnt, pa, sz, exflags);
 }
 
 size_t
 physmem_avail(vm_paddr_t *avail, size_t maxavail)
 {
 
 	return (regions_to_avail(avail, EXFLAG_NOALLOC, maxavail, 0, NULL, NULL));
 }
 
 bool
 physmem_excluded(vm_paddr_t pa, vm_size_t sz)
 {
 	const struct region *exp;
 	size_t exi;
 
 	for (exi = 0, exp = exregions; exi < excnt; ++exi, ++exp) {
 		if (pa < exp->addr || pa + sz > exp->addr + exp->size)
 			continue;
 		return (true);
 	}
 	return (false);
 }
 
 #ifdef _KERNEL
 /*
  * Process all the regions added earlier into the global avail lists.
  *
  * Updates the kernel global 'physmem' with the number of physical pages
  * available for use (all pages not in any exclusion region).
  *
  * Updates the kernel global 'Maxmem' with the page number one greater then the
  * last page of physical memory in the system.
  */
 void
 physmem_init_kernel_globals(void)
 {
 	size_t nextidx;
 	u_long hwphyssz;
 
 	hwphyssz = 0;
 	TUNABLE_ULONG_FETCH("hw.physmem", &hwphyssz);
 
 	regions_to_avail(dump_avail, EXFLAG_NODUMP, PHYS_AVAIL_ENTRIES,
 	    hwphyssz, NULL, NULL);
 	nextidx = regions_to_avail(phys_avail, EXFLAG_NOALLOC,
 	    PHYS_AVAIL_ENTRIES, hwphyssz, &physmem, &realmem);
 	if (nextidx == 0)
 		panic("No memory entries in phys_avail");
 	Maxmem = atop(phys_avail[nextidx - 1]);
 }
 
 #ifdef DDB
 #include <ddb/ddb.h>
 
 DB_SHOW_COMMAND_FLAGS(physmem, db_show_physmem, DB_CMD_MEMSAFE)
 {
 
 	physmem_dump_tables(db_printf);
 }
 
 #endif /* DDB */
 
 /*
  * ram pseudo driver - this reserves I/O space resources corresponding to physical
  * memory regions.
  */
 
 static void
 ram_identify(driver_t *driver, device_t parent)
 {
 
 	if (resource_disabled("ram", 0))
 		return;
 	if (BUS_ADD_CHILD(parent, 0, "ram", 0) == NULL)
 		panic("ram_identify");
 }
 
 static int
 ram_probe(device_t dev)
 {
 
 	device_quiet(dev);
 	device_set_desc(dev, "System RAM");
 	return (BUS_PROBE_SPECIFIC);
 }
 
 static int
 ram_attach(device_t dev)
 {
 	vm_paddr_t avail_list[PHYS_AVAIL_COUNT];
 	rman_res_t start, end;
 	struct region *hwp;
 	int rid, i;
 
 	rid = 0;
 
 	/* Get the avail list. */
 	bzero(avail_list, sizeof(avail_list));
 	regions_to_avail(avail_list, EXFLAG_NOALLOC | EXFLAG_NODUMP,
 	    PHYS_AVAIL_COUNT, 0, NULL, NULL);
 
 	/* Reserve all memory regions. */
 	for (i = 0; avail_list[i + 1] != 0; i += 2) {
 		start = avail_list[i];
 		end = avail_list[i + 1];
 
 		if (bootverbose)
 			device_printf(dev,
 			    "reserving memory region:   %jx-%jx\n",
 			    (uintmax_t)start, (uintmax_t)end);
 
 		if (bus_alloc_resource(dev, SYS_RES_MEMORY, &rid, start, end,
 		    end - start, 0) == NULL)
 			panic("ram_attach: resource %d failed to attach", rid);
 		rid++;
 	}
 
 	/* Now, reserve the excluded memory regions. */
 	for (i = 0, hwp = exregions; i < excnt; i++, hwp++) {
 		start = hwp->addr;
 		end = hwp->addr + hwp->size;
 
 		if (bootverbose)
 			device_printf(dev,
 			    "reserving excluded region: %jx-%jx\n",
 			    (uintmax_t)start, (uintmax_t)(end - 1));
 
 		/*
 		 * Best-effort attempt to reserve the range. This may fail, as
 		 * sometimes the excluded ranges provided by the device tree
 		 * will cover or overlap some I/O range.
 		 */
 		if (bus_alloc_resource(dev, SYS_RES_MEMORY, &rid, start, end,
 		    end - start, 0) == NULL) {
 			if (bootverbose)
 				device_printf(dev, "failed to reserve region\n");
 			continue;
 		}
 		rid++;
 	}
 
 	return (0);
 }
 
 static device_method_t ram_methods[] = {
 	/* Device interface */
 	DEVMETHOD(device_identify,	ram_identify),
 	DEVMETHOD(device_probe,		ram_probe),
 	DEVMETHOD(device_attach,	ram_attach),
 
 	DEVMETHOD_END
 };
 
 DEFINE_CLASS_0(ram, ram_driver, ram_methods, /* no softc */ 1);
 DRIVER_MODULE(ram, nexus, ram_driver, 0, 0);
 #endif /* _KERNEL */