diff --git a/stubdom/grub/kexec.c b/stubdom/grub/kexec.c
index 0b2f4f3ddb..2300318b80 100644
--- a/stubdom/grub/kexec.c
+++ b/stubdom/grub/kexec.c
@@ -100,9 +100,9 @@ static void do_exchange(struct xc_dom_image *dom, xen_pfn_t target_pfn, xen_pfn_
     dom->p2m_host[target_pfn] = source_mfn;
 }
 
-int kexec_allocate(struct xc_dom_image *dom, xen_vaddr_t up_to)
+int kexec_allocate(struct xc_dom_image *dom)
 {
-    unsigned long new_allocated = (up_to - dom->parms.virt_base) / PAGE_SIZE;
+    unsigned long new_allocated = dom->pfn_alloc_end - dom->rambase_pfn;
     unsigned long i;
 
     pages = realloc(pages, new_allocated * sizeof(*pages));
@@ -319,8 +319,6 @@ void kexec(void *kernel, long kernel_size, void *module, long module_size, char
 
     /* Make sure the bootstrap page table does not RW-map any of our current
      * page table frames */
-    kexec_allocate(dom, dom->virt_pgtab_end);
-
     if ( (rc = xc_dom_update_guest_p2m(dom))) {
         grub_printf("xc_dom_update_guest_p2m returned %d\n", rc);
         errnum = ERR_BOOT_FAILURE;
diff --git a/tools/libxc/include/xc_dom.h b/tools/libxc/include/xc_dom.h
index 6c15589402..3a0aa491fb 100644
--- a/tools/libxc/include/xc_dom.h
+++ b/tools/libxc/include/xc_dom.h
@@ -32,6 +32,7 @@ struct xc_dom_seg {
     xen_vaddr_t vstart;
     xen_vaddr_t vend;
     xen_pfn_t pfn;
+    xen_pfn_t pages;
 };
 
 struct xc_dom_mem {
@@ -93,6 +94,7 @@ struct xc_dom_image {
     xen_pfn_t xenstore_pfn;
     xen_pfn_t shared_info_pfn;
     xen_pfn_t bootstack_pfn;
+    xen_pfn_t pfn_alloc_end;
     xen_vaddr_t virt_alloc_end;
     xen_vaddr_t bsd_symtab_start;
 
@@ -178,8 +180,8 @@ struct xc_dom_image {
 
     /* kernel loader */
     struct xc_dom_arch *arch_hooks;
-    /* allocate up to virt_alloc_end */
-    int (*allocate) (struct xc_dom_image * dom, xen_vaddr_t up_to);
+    /* allocate up to pfn_alloc_end */
+    int (*allocate) (struct xc_dom_image * dom);
 
     /* Container type (HVM or PV). */
     enum {
@@ -363,14 +365,11 @@ static inline void *xc_dom_seg_to_ptr_pages(struct xc_dom_image *dom,
                                       struct xc_dom_seg *seg,
                                       xen_pfn_t *pages_out)
 {
-    xen_vaddr_t segsize = seg->vend - seg->vstart;
-    unsigned int page_size = XC_DOM_PAGE_SIZE(dom);
-    xen_pfn_t pages = (segsize + page_size - 1) / page_size;
     void *retval;
 
-    retval = xc_dom_pfn_to_ptr(dom, seg->pfn, pages);
+    retval = xc_dom_pfn_to_ptr(dom, seg->pfn, seg->pages);
 
-    *pages_out = retval ? pages : 0;
+    *pages_out = retval ? seg->pages : 0;
     return retval;
 }
 
diff --git a/tools/libxc/xc_dom_core.c b/tools/libxc/xc_dom_core.c
index fbe4464c3a..a14d477cda 100644
--- a/tools/libxc/xc_dom_core.c
+++ b/tools/libxc/xc_dom_core.c
@@ -535,56 +535,75 @@ void *xc_dom_pfn_to_ptr_retcount(struct xc_dom_image *dom, xen_pfn_t pfn,
     return phys->ptr;
 }
 
+static int xc_dom_chk_alloc_pages(struct xc_dom_image *dom, char *name,
+                                  xen_pfn_t pages)
+{
+    unsigned int page_size = XC_DOM_PAGE_SIZE(dom);
+
+    if ( pages > dom->total_pages || /* multiple test avoids overflow probs */
+         dom->pfn_alloc_end - dom->rambase_pfn > dom->total_pages ||
+         pages > dom->total_pages - dom->pfn_alloc_end + dom->rambase_pfn )
+    {
+        xc_dom_panic(dom->xch, XC_OUT_OF_MEMORY,
+                     "%s: segment %s too large (0x%"PRIpfn" > "
+                     "0x%"PRIpfn" - 0x%"PRIpfn" pages)", __FUNCTION__, name,
+                     pages, dom->total_pages,
+                     dom->pfn_alloc_end - dom->rambase_pfn);
+        return -1;
+    }
+
+    dom->pfn_alloc_end += pages;
+    dom->virt_alloc_end += pages * page_size;
+
+    return 0;
+}
+
+static int xc_dom_alloc_pad(struct xc_dom_image *dom, xen_vaddr_t boundary)
+{
+    unsigned int page_size = XC_DOM_PAGE_SIZE(dom);
+    xen_pfn_t pages;
+
+    if ( boundary & (page_size - 1) )
+    {
+        xc_dom_panic(dom->xch, XC_INTERNAL_ERROR,
+                     "%s: segment boundary isn't page aligned (0x%" PRIx64 ")",
+                     __FUNCTION__, boundary);
+        return -1;
+    }
+    if ( boundary < dom->virt_alloc_end )
+    {
+        xc_dom_panic(dom->xch, XC_INTERNAL_ERROR,
+                     "%s: segment boundary too low (0x%" PRIx64 " < 0x%" PRIx64
+                     ")", __FUNCTION__, boundary, dom->virt_alloc_end);
+        return -1;
+    }
+    pages = (boundary - dom->virt_alloc_end) / page_size;
+
+    return xc_dom_chk_alloc_pages(dom, "padding", pages);
+}
+
 int xc_dom_alloc_segment(struct xc_dom_image *dom,
                          struct xc_dom_seg *seg, char *name,
                          xen_vaddr_t start, xen_vaddr_t size)
 {
     unsigned int page_size = XC_DOM_PAGE_SIZE(dom);
-    xen_pfn_t pages = (size + page_size - 1) / page_size;
-    xen_pfn_t pfn;
+    xen_pfn_t pages;
     void *ptr;
 
-    if ( start == 0 )
-        start = dom->virt_alloc_end;
-
-    if ( start & (page_size - 1) )
-    {
-        xc_dom_panic(dom->xch, XC_INTERNAL_ERROR,
-                     "%s: segment start isn't page aligned (0x%" PRIx64 ")",
-                     __FUNCTION__, start);
+    if ( start && xc_dom_alloc_pad(dom, start) )
         return -1;
-    }
-    if ( start < dom->virt_alloc_end )
-    {
-        xc_dom_panic(dom->xch, XC_INTERNAL_ERROR,
-                     "%s: segment start too low (0x%" PRIx64 " < 0x%" PRIx64
-                     ")", __FUNCTION__, start, dom->virt_alloc_end);
+
+    pages = (size + page_size - 1) / page_size;
+    start = dom->virt_alloc_end;
+
+    seg->pfn = dom->pfn_alloc_end;
+    seg->pages = pages;
+
+    if ( xc_dom_chk_alloc_pages(dom, name, pages) )
         return -1;
-    }
 
-    seg->vstart = start;
-    pfn = (seg->vstart - dom->parms.virt_base) / page_size;
-    seg->pfn = pfn + dom->rambase_pfn;
-
-    if ( pages > dom->total_pages || /* multiple test avoids overflow probs */
-         pfn > dom->total_pages ||
-         pages > dom->total_pages - pfn)
-    {
-        xc_dom_panic(dom->xch, XC_OUT_OF_MEMORY,
-                     "%s: segment %s too large (0x%"PRIpfn" > "
-                     "0x%"PRIpfn" - 0x%"PRIpfn" pages)",
-                     __FUNCTION__, name, pages, dom->total_pages, pfn);
-        return -1;
-    }
-
-    seg->vend = start + pages * page_size;
-    dom->virt_alloc_end = seg->vend;
     if (dom->allocate)
-        dom->allocate(dom, dom->virt_alloc_end);
-
-    DOMPRINTF("%-20s:   %-12s : 0x%" PRIx64 " -> 0x%" PRIx64
-              "  (pfn 0x%" PRIpfn " + 0x%" PRIpfn " pages)",
-              __FUNCTION__, name, seg->vstart, seg->vend, seg->pfn, pages);
+        dom->allocate(dom);
 
     /* map and clear pages */
     ptr = xc_dom_seg_to_ptr(dom, seg);
@@ -592,6 +611,13 @@ int xc_dom_alloc_segment(struct xc_dom_image *dom,
         return -1;
     memset(ptr, 0, pages * page_size);
 
+    seg->vstart = start;
+    seg->vend = dom->virt_alloc_end;
+
+    DOMPRINTF("%-20s:   %-12s : 0x%" PRIx64 " -> 0x%" PRIx64
+              "  (pfn 0x%" PRIpfn " + 0x%" PRIpfn " pages)",
+              __FUNCTION__, name, seg->vstart, seg->vend, seg->pfn, pages);
+
     return 0;
 }
 
@@ -602,10 +628,11 @@ int xc_dom_alloc_page(struct xc_dom_image *dom, char *name)
     xen_pfn_t pfn;
 
     start = dom->virt_alloc_end;
+    pfn = dom->pfn_alloc_end - dom->rambase_pfn;
     dom->virt_alloc_end += page_size;
-    if (dom->allocate)
-        dom->allocate(dom, dom->virt_alloc_end);
-    pfn = (start - dom->parms.virt_base) / page_size;
+    dom->pfn_alloc_end++;
+    if ( dom->allocate )
+        dom->allocate(dom);
 
     DOMPRINTF("%-20s:   %-12s : 0x%" PRIx64 " (pfn 0x%" PRIpfn ")",
               __FUNCTION__, name, start, pfn);
@@ -886,6 +913,7 @@ int xc_dom_parse_image(struct xc_dom_image *dom)
 int xc_dom_rambase_init(struct xc_dom_image *dom, uint64_t rambase)
 {
     dom->rambase_pfn = rambase >> XC_PAGE_SHIFT;
+    dom->pfn_alloc_end = dom->rambase_pfn;
     DOMPRINTF("%s: RAM starts at %"PRI_xen_pfn,
               __FUNCTION__, dom->rambase_pfn);
     return 0;
@@ -1013,6 +1041,8 @@ int xc_dom_build_image(struct xc_dom_image *dom)
         goto err;
     }
     page_size = XC_DOM_PAGE_SIZE(dom);
+    if ( dom->parms.virt_base != UNSET_ADDR )
+        dom->virt_alloc_end = dom->parms.virt_base;
 
     /* load kernel */
     if ( xc_dom_alloc_segment(dom, &dom->kernel_seg, "kernel",
@@ -1067,6 +1097,11 @@ int xc_dom_build_image(struct xc_dom_image *dom)
               __FUNCTION__, dom->virt_alloc_end);
     DOMPRINTF("%-20s: virt_pgtab_end : 0x%" PRIx64 "",
               __FUNCTION__, dom->virt_pgtab_end);
+
+    /* Make sure all memory mapped by initial page tables is available */
+    if ( dom->virt_pgtab_end && xc_dom_alloc_pad(dom, dom->virt_pgtab_end) )
+        return -1;
+
     return 0;
 
  err: