diff --git a/usr.sbin/bhyve/pci_emul.h b/usr.sbin/bhyve/pci_emul.h
--- a/usr.sbin/bhyve/pci_emul.h
+++ b/usr.sbin/bhyve/pci_emul.h
@@ -97,6 +97,7 @@
 	uint64_t		size;
 	uint64_t		addr;
 	uint8_t			lobits;
+	uint64_t		mapped_size;
 };
 
 #define PI_NAMESZ	40
diff --git a/usr.sbin/bhyve/pci_passthru.c b/usr.sbin/bhyve/pci_passthru.c
--- a/usr.sbin/bhyve/pci_passthru.c
+++ b/usr.sbin/bhyve/pci_passthru.c
@@ -77,6 +77,8 @@
 
 static int pcifd = -1;
 
+static void ppt_build_occupied_pages(void);
+
 SET_DECLARE(passthru_dev_set, struct passthru_dev);
 
 struct passthru_softc {
@@ -564,6 +566,141 @@
 	return (0);
 }
 
+struct occ_entry {
+	uint64_t page;
+	uint32_t bdfd;
+	uint8_t bar_idx;
+};
+
+static int g_occ_built = 0;
+static struct occ_entry *g_occ;
+static size_t g_occ_n, g_occ_cap;
+
+static inline uint32_t
+pack_bdfd(struct pcisel s)
+{
+	return ((uint32_t)s.pc_domain << 16) | ((uint32_t)s.pc_bus << 8) |
+	    ((uint32_t)s.pc_dev << 3) | (uint32_t)s.pc_func;
+}
+
+static void
+occ_push(uint64_t page, uint32_t bdfd, uint8_t bar_idx)
+{
+	if (g_occ_n == g_occ_cap) {
+		size_t nc = g_occ_cap ? g_occ_cap * 2 : 256;
+		void *nv = realloc(g_occ, nc * sizeof(*g_occ));
+		g_occ = nv;
+		g_occ_cap = nc;
+	}
+	g_occ[g_occ_n++] = (struct occ_entry) {
+		.page = page,
+		.bdfd = bdfd,
+		.bar_idx = bar_idx
+	};
+}
+
+static int
+cmp_occ(const void *a, const void *b)
+{
+	const struct occ_entry *x = a, *y = b;
+
+	if (x->page < y->page)
+		return -1;
+	if (x->page > y->page)
+		return 1;
+	if (x->bdfd < y->bdfd)
+		return -1;
+	if (x->bdfd > y->bdfd)
+		return 1;
+	return 0;
+}
+
+static int
+page_has_other_dev(uint64_t page, uint32_t self_bdfd)
+{
+	size_t lo = 0, hi = g_occ_n;
+
+	while (lo < hi) {
+		size_t mid = (lo + hi) / 2;
+		if (g_occ[mid].page < page)
+			lo = mid + 1;
+		else
+			hi = mid;
+	}
+	for (size_t i = lo; i < g_occ_n && g_occ[i].page == page; ++i) {
+		if (g_occ[i].bdfd != self_bdfd)
+			return 1;
+	}
+	return 0;
+}
+
+static void
+ppt_build_occupied_pages(void)
+{
+	struct pci_conf_io cio;
+	struct pci_conf *buf = NULL;
+	const size_t step = 256;
+
+	bzero(&cio, sizeof(cio));
+	buf = calloc(step, sizeof(*buf));
+
+	cio.matches = buf;
+	cio.match_buf_len = step * sizeof(*buf);
+
+	for (;;) {
+		cio.num_matches = 0;
+		if (ioctl(pcifd, PCIOCGETCONF, &cio) < 0)
+			err(1, "PCIOCGETCONF");
+
+		for (unsigned i = 0; i < cio.num_matches; i++) {
+			struct pci_bar_io bar;
+			uint32_t bdfd = pack_bdfd(buf[i].pc_sel);
+
+			for (int r = 0; r <= PCI_BARMAX; r++) {
+				bzero(&bar, sizeof(bar));
+				bar.pbi_sel = buf[i].pc_sel;
+				bar.pbi_reg = PCIR_BAR(r);
+				if (ioctl(pcifd, PCIOCGETBAR, &bar) < 0)
+					continue;
+
+				if (PCI_BAR_IO(bar.pbi_base))
+					continue;
+
+				uint64_t base = bar.pbi_base &
+				    PCIM_BAR_MEM_BASE;
+				uint64_t size = bar.pbi_length;
+				if (size == 0)
+					continue;
+
+				uint64_t start = base & ~PAGE_MASK;
+				uint64_t end = (base + size + PAGE_MASK) &
+				    ~PAGE_MASK;
+				for (uint64_t p = start; p < end;
+				    p += PAGE_SIZE)
+					occ_push(p, bdfd, (uint8_t)r);
+
+				if ((bar.pbi_base & PCIM_BAR_MEM_TYPE) ==
+				    PCIM_BAR_MEM_64)
+					r++;
+			}
+		}
+		if (cio.status == PCI_GETCONF_LAST_DEVICE)
+			break;
+		cio.offset += cio.num_matches * sizeof(struct pci_conf);
+	}
+	qsort(g_occ, g_occ_n, sizeof(g_occ[0]), cmp_occ);
+	free(buf);
+}
+
+static inline void
+build_occ_map(void)
+{
+	if (!g_occ_built) {
+		ppt_build_occupied_pages();
+		g_occ_built = 1;
+	}
+}
+
 static int
 cfginitbar(struct passthru_softc *sc)
 {
@@ -572,9 +709,12 @@
 	struct pci_bar_io bar;
 	enum pcibar_type bartype;
 	uint64_t base, size;
+	uint32_t self_bdf;
 
 	pi = sc->psc_pi;
-
+	self_bdf = ((uint32_t)sc->psc_sel.pc_bus << 8) |
+		((uint32_t)sc->psc_sel.pc_dev << 3) |
+		(uint32_t)sc->psc_sel.pc_func;
 	/*
 	 * Initialize BAR registers
 	 */
@@ -604,21 +744,44 @@
 		}
 		size = bar.pbi_length;
 
-		if (bartype != PCIBAR_IO) {
-			if (((base | size) & PAGE_MASK) != 0) {
-				warnx("passthru device %d/%d/%d BAR %d: "
-				    "base %#lx or size %#lx not page aligned\n",
-				    sc->psc_sel.pc_bus, sc->psc_sel.pc_dev,
-				    sc->psc_sel.pc_func, i, base, size);
-				return (-1);
-			}
-		}
-
 		/* Cache information about the "real" BAR */
 		sc->psc_bar[i].type = bartype;
 		sc->psc_bar[i].size = size;
 		sc->psc_bar[i].addr = base;
 		sc->psc_bar[i].lobits = 0;
+		sc->psc_bar[i].mapped_size = size;
+
+		if (bartype != PCIBAR_IO) {
+			if ((base & PAGE_MASK) != 0) {
+					warnx("passthru device %d/%d/%d BAR %d: base %#lx not page aligned",
+						sc->psc_sel.pc_bus, sc->psc_sel.pc_dev,
+						sc->psc_sel.pc_func, i, base);
+					return (-1);
+			}
+			if ((size & PAGE_MASK) != 0) {
+				if (size < PAGE_SIZE) {
+					uint64_t page = base & ~PAGE_MASK;
+					build_occ_map();
+					if (page_has_other_dev(page, self_bdf)) {
+						warnx("passthru device %d/%d/%d BAR %d: "
+							"size %#lx <4K but page %#lx shared by other device",
+							sc->psc_sel.pc_bus,
+							sc->psc_sel.pc_dev,
+							sc->psc_sel.pc_func, i,
+							size, page);
+						return (-1);
+					}
+					sc->psc_bar[i].mapped_size = PAGE_SIZE;
+				} else {
+					warnx("passthru device %d/%d/%d BAR %d: "
+						"size %#lx not multiple of page size",
+						sc->psc_sel.pc_bus,
+						sc->psc_sel.pc_dev,
+						sc->psc_sel.pc_func, i, size);
+					return (-1);
+				}
+			}
+		}
 
 		/* Allocate the BAR in the guest I/O or MMIO space */
 		error = pci_emul_alloc_bar(pi, i, bartype, size);
@@ -1283,13 +1446,13 @@
 		if (vm_unmap_pptdev_mmio(pi->pi_vmctx, sc->psc_sel.pc_bus,
 					 sc->psc_sel.pc_dev,
 					 sc->psc_sel.pc_func, address,
-					 sc->psc_bar[baridx].size) != 0)
+					 sc->psc_bar[baridx].mapped_size) != 0)
 			warnx("pci_passthru: unmap_pptdev_mmio failed");
 	} else {
 		if (vm_map_pptdev_mmio(pi->pi_vmctx, sc->psc_sel.pc_bus,
 				       sc->psc_sel.pc_dev,
 				       sc->psc_sel.pc_func, address,
-				       sc->psc_bar[baridx].size,
+				       sc->psc_bar[baridx].mapped_size	,
 				       sc->psc_bar[baridx].addr) != 0)
 			warnx("pci_passthru: map_pptdev_mmio failed");
 	}