[Xen-changelog] [xen-unstable] hvm: Allocate an unmapped guard page to separate ordinary low RAM

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1200574750 0
# Node ID be960a84e492c1baa0a2dfa76b1a770d0df5e433
# Parent  f17b34df974f2c65fd10262a223e8246545802fc
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@xxxxxxxxxx>
---
 tools/libxc/xc_hvm_build.c |   21 ++++++++++++++++++---
 1 files changed, 18 insertions(+), 3 deletions(-)

diff -r f17b34df974f -r be960a84e492 tools/libxc/xc_hvm_build.c
--- a/tools/libxc/xc_hvm_build.c        Wed Jan 16 15:55:57 2008 +0000
+++ b/tools/libxc/xc_hvm_build.c        Thu Jan 17 12:59:10 2008 +0000
@@ -77,13 +77,19 @@ static void build_e820map(void *e820_pag
     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;
@@ -244,6 +250,15 @@ 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) ||

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