--- a/xen/arch/x86/efi/stub.c
+++ b/xen/arch/x86/efi/stub.c
@@ -52,6 +52,12 @@ bool efi_enabled(unsigned int feature)
 
 void __init efi_init_memory(void) { }
 
+bool efi_boot_mem_unused(unsigned long *start, unsigned long *end)
+{
+    *start = *end = (unsigned long)_end;
+    return false;
+}
+
 void efi_update_l4_pgtable(unsigned int l4idx, l4_pgentry_t l4e) { }
 
 bool efi_rs_using_pgtables(void)
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -830,6 +830,7 @@ void __init noreturn __start_xen(unsigne
     module_t *mod;
     unsigned long nr_pages, raw_max_page, modules_headroom, module_map[1];
     int i, j, e820_warn = 0, bytes = 0;
+    unsigned long eb_start, eb_end;
     bool acpi_boot_table_init_done = false, relocated = false;
     int ret;
     struct ns16550_defaults ns16550 = {
@@ -1145,7 +1146,8 @@ void __init noreturn __start_xen(unsigne
 
         /*
          * This needs to remain in sync with xen_in_range() and the
-         * respective reserve_e820_ram() invocation below.
+         * respective reserve_e820_ram() invocation below. No need to
+         * query efi_boot_mem_unused() here, though.
          */
         mod[mbi->mods_count].mod_start = virt_to_mfn(_stext);
         mod[mbi->mods_count].mod_end = __2M_rwdata_end - _stext;
@@ -1417,8 +1419,18 @@ void __init noreturn __start_xen(unsigne
     if ( !xen_phys_start )
         panic("Not enough memory to relocate Xen\n");
 
+    /* FIXME: Putting a hole in .bss would shatter the large page mapping. */
+    if ( using_2M_mapping() )
+        efi_boot_mem_unused(NULL, NULL);
+
     /* This needs to remain in sync with xen_in_range(). */
-    reserve_e820_ram(&boot_e820, __pa(_stext), __pa(__2M_rwdata_end));
+    if ( efi_boot_mem_unused(&eb_start, &eb_end) )
+    {
+        reserve_e820_ram(&boot_e820, __pa(_stext), __pa(eb_start));
+        reserve_e820_ram(&boot_e820, __pa(eb_end), __pa(__2M_rwdata_end));
+    }
+    else
+        reserve_e820_ram(&boot_e820, __pa(_stext), __pa(__2M_rwdata_end));
 
     /* Late kexec reservation (dynamic start address). */
     kexec_reserve_area(&boot_e820);
@@ -1979,7 +1991,7 @@ int __hwdom_init xen_in_range(unsigned l
     paddr_t start, end;
     int i;
 
-    enum { region_s3, region_ro, region_rw, nr_regions };
+    enum { region_s3, region_ro, region_rw, region_bss, nr_regions };
     static struct {
         paddr_t s, e;
     } xen_regions[nr_regions] __hwdom_initdata;
@@ -2004,6 +2016,14 @@ int __hwdom_init xen_in_range(unsigned l
         /* hypervisor .data + .bss */
         xen_regions[region_rw].s = __pa(&__2M_rwdata_start);
         xen_regions[region_rw].e = __pa(&__2M_rwdata_end);
+        if ( efi_boot_mem_unused(&start, &end) )
+        {
+            ASSERT(__pa(start) >= xen_regions[region_rw].s);
+            ASSERT(__pa(end) <= xen_regions[region_rw].e);
+            xen_regions[region_rw].e = __pa(start);
+            xen_regions[region_bss].s = __pa(end);
+            xen_regions[region_bss].e = __pa(&__2M_rwdata_end);
+        }
     }
 
     start = (paddr_t)mfn << PAGE_SHIFT;
--- a/xen/arch/x86/tboot.c
+++ b/xen/arch/x86/tboot.c
@@ -1,3 +1,4 @@
+#include <xen/efi.h>
 #include <xen/init.h>
 #include <xen/types.h>
 #include <xen/lib.h>
@@ -364,6 +365,8 @@ void tboot_shutdown(uint32_t shutdown_ty
     /* if this is S3 then set regions to MAC */
     if ( shutdown_type == TB_SHUTDOWN_S3 )
     {
+        unsigned long s, e;
+
         /*
          * Xen regions for tboot to MAC. This needs to remain in sync with
          * xen_in_range().
@@ -378,6 +381,15 @@ void tboot_shutdown(uint32_t shutdown_ty
         /* hypervisor .data + .bss */
         g_tboot_shared->mac_regions[2].start = (uint64_t)__pa(&__2M_rwdata_start);
         g_tboot_shared->mac_regions[2].size = __2M_rwdata_end - __2M_rwdata_start;
+        if ( efi_boot_mem_unused(&s, &e) )
+        {
+            g_tboot_shared->mac_regions[2].size =
+                s - (unsigned long)__2M_rwdata_start;
+            g_tboot_shared->mac_regions[3].start = __pa(e);
+            g_tboot_shared->mac_regions[3].size =
+                (unsigned long)__2M_rwdata_end - e;
+            g_tboot_shared->num_mac_regions = 4;
+        }
 
         /*
          * MAC domains and other Xen memory
--- a/xen/common/efi/ebmalloc.c
+++ b/xen/common/efi/ebmalloc.c
@@ -1,5 +1,6 @@
 #include "efi.h"
 #include <xen/init.h>
+#include <xen/mm.h>
 
 #ifdef CONFIG_ARM
 /*
@@ -21,7 +22,7 @@
 
 static char __section(".bss.page_aligned") __aligned(PAGE_SIZE)
     ebmalloc_mem[EBMALLOC_SIZE];
-static unsigned long __initdata ebmalloc_allocated;
+static unsigned long __read_mostly ebmalloc_allocated;
 
 /* EFI boot allocator. */
 void __init *ebmalloc(size_t size)
@@ -36,17 +37,37 @@ void __init *ebmalloc(size_t size)
     return ptr;
 }
 
+bool efi_boot_mem_unused(unsigned long *start, unsigned long *end)
+{
+    if ( !start && !end )
+    {
+        ebmalloc_allocated = sizeof(ebmalloc_mem);
+        return false;
+    }
+
+    *start = (unsigned long)ebmalloc_mem + PAGE_ALIGN(ebmalloc_allocated);
+    *end = (unsigned long)ebmalloc_mem + sizeof(ebmalloc_mem);
+
+    return *start < *end;
+}
+
 void __init free_ebmalloc_unused_mem(void)
 {
-#if 0 /* FIXME: Putting a hole in the BSS breaks the IOMMU mappings for dom0. */
     unsigned long start, end;
 
-    start = (unsigned long)ebmalloc_mem + PAGE_ALIGN(ebmalloc_allocated);
-    end = (unsigned long)ebmalloc_mem + sizeof(ebmalloc_mem);
+    if ( !efi_boot_mem_unused(&start, &end) )
+        return;
 
     destroy_xen_mappings(start, end);
+
+#ifdef CONFIG_X86
+    /*
+     * By reserving the space early in the E820 map, it gets freed way before
+     * we make it here. Don't free the range a 2nd time.
+     */
+#else
     init_xenheap_pages(__pa(start), __pa(end));
+#endif
 
     printk(XENLOG_INFO "Freed %lukB unused BSS memory\n", (end - start) >> 10);
-#endif
 }
--- a/xen/include/xen/efi.h
+++ b/xen/include/xen/efi.h
@@ -33,6 +33,7 @@ struct compat_pf_efi_runtime_call;
 
 bool efi_enabled(unsigned int feature);
 void efi_init_memory(void);
+bool efi_boot_mem_unused(unsigned long *start, unsigned long *end);
 bool efi_rs_using_pgtables(void);
 unsigned long efi_get_time(void);
 void efi_halt_system(void);
