hvm: Allocate an unmapped guard page to separate ordinary low RAM

from special pages (ioreq, xenstore, etc). This will stop some probe
functions (e.g., embedded in memtest) from stomping on the special
pages.
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>
xen-unstable changeset:   16728:be960a84e492c1baa0a2dfa76b1a770d0df5e433
xen-unstable date:        Thu Jan 17 12:59:10 2008 +0000

diff --git a/tools/libxc/xc_hvm_build.c b/tools/libxc/xc_hvm_build.c
index f3a0707a31678e252f38e05ee08de3fc7a14b30c..71126abcbad61251e93ee6eb62b5d767e8e49cdc 100644 (file)
--- a/tools/libxc/xc_hvm_build.c
+++ b/tools/libxc/xc_hvm_build.c
@@ -85,13 +85,19 @@ static void build_e820map(void *e820_page, unsigned long long mem_size)
     e820entry[nr_map].type = E820_RESERVED;
     nr_map++;
 
-    /* Low RAM goes here. Remove 3 pages for ioreq, bufioreq, and xenstore. */
+    /*
+     * Low RAM goes here. Remove 4 pages for: ioreq, bufioreq, and xenstore.
+     *  1. Guard page.
+     *  2. Buffered ioreq.
+     *  3. Xenstore.
+     *  4. Normal ioreq.
+     */
     e820entry[nr_map].addr = 0x100000;
-    e820entry[nr_map].size = mem_size - 0x100000 - PAGE_SIZE * 3;
+    e820entry[nr_map].size = mem_size - 0x100000 - PAGE_SIZE * 4;
     e820entry[nr_map].type = E820_RAM;
     nr_map++;
 
-    /* Explicitly reserve space for special pages (ioreq and xenstore). */
+    /* Explicitly reserve space for special pages. */
     e820entry[nr_map].addr = mem_size - PAGE_SIZE * 3;
     e820entry[nr_map].size = PAGE_SIZE * 3;
     e820entry[nr_map].type = E820_RESERVED;
@@ -253,6 +259,15 @@ static int setup_guest(int xc_handle,
     else
         shared_page_nr = (v_end >> PAGE_SHIFT) - 1;
 
+    /* Free the guard page that separates low RAM from special pages. */
+    rc = xc_domain_memory_decrease_reservation(
+            xc_handle, dom, 1, 0, &page_array[shared_page_nr-3]);
+    if ( rc != 0 )
+    {
+        PERROR("Could not deallocate guard page for HVM guest.\n");
+        goto error_out;
+    }
+
     /* Paranoia: clean pages. */
     if ( xc_clear_domain_page(xc_handle, dom, shared_page_nr) ||
          xc_clear_domain_page(xc_handle, dom, shared_page_nr-1) ||