1 files changed, 11 insertions, 7 deletions

diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index b1dbdaa23ecc..769c4b01fa32 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -118,16 +118,18 @@ static unsigned long __init xen_return_unused_memory(unsigned long max_pfn,
                                                      const struct e820map *e820)
 {
         phys_addr_t max_addr = PFN_PHYS(max_pfn);
-        phys_addr_t last_end = 0;
+        phys_addr_t last_end = ISA_END_ADDRESS;
         unsigned long released = 0;
         int i;
 
+        /* Free any unused memory above the low 1Mbyte. */
         for (i = 0; i < e820->nr_map && last_end < max_addr; i++) {
                 phys_addr_t end = e820->map[i].addr;
                 end = min(max_addr, end);
 
-                released += xen_release_chunk(last_end, end);
-                last_end = e820->map[i].addr + e820->map[i].size;
+                if (last_end < end)
+                        released += xen_release_chunk(last_end, end);
+                last_end = max(last_end, e820->map[i].addr + e820->map[i].size);
         }
 
         if (last_end < max_addr)
@@ -164,6 +166,7 @@ char * __init xen_memory_setup(void)
                 XENMEM_memory_map;
         rc = HYPERVISOR_memory_op(op, &memmap);
         if (rc == -ENOSYS) {
+                BUG_ON(xen_initial_domain());
                 memmap.nr_entries = 1;
                 map[0].addr = 0ULL;
                 map[0].size = mem_end;
@@ -201,12 +204,13 @@ char * __init xen_memory_setup(void)
         }
 
         /*
-         * Even though this is normal, usable memory under Xen, reserve
-         * ISA memory anyway because too many things think they can poke
+         * In domU, the ISA region is normal, usable memory, but we
+         * reserve ISA memory anyway because too many things poke
          * about in there.
          *
-         * In a dom0 kernel, this region is identity mapped with the
-         * hardware ISA area, so it really is out of bounds.
+         * In Dom0, the host E820 information can leave gaps in the
+         * ISA range, which would cause us to release those pages.  To
+         * avoid this, we unconditionally reserve them here.
          */
         e820_add_region(ISA_START_ADDRESS, ISA_END_ADDRESS - ISA_START_ADDRESS,
                         E820_RESERVED);