permute with 2MB chunk

diff -r 59b8768d0d0d tools/libxc/xc_domain_save.c
@@ -125,34 +125,22 @@ static inline int count_bits ( int nr, v
     return count;
 }
 
-static inline int permute( int i, int nr, int order_nr  )
+static inline int permute(unsigned long i, unsigned long order_nr)
 {
     /* Need a simple permutation function so that we scan pages in a
        pseudo random order, enabling us to get a better estimate of
        the domain's page dirtying rate as we go (there are often
        contiguous ranges of pfns that have similar behaviour, and we
        want to mix them up. */
+  
+  unsigned char keep = 9; /* chunk of 2 MB */
+  unsigned char shift_low = (order_nr - keep) / 2 + ((order_nr - keep) / 2) % 2;
+  unsigned char shift_high = order_nr - keep - shift_low;
 
-    /* e.g. nr->oder 15->4 16->4 17->5 */
-    /* 512MB domain, 128k pages, order 17 */
+  unsigned long high = (i >> (keep + shift_low));
+  unsigned long low = (i >> keep) & ((1 << shift_low) - 1);
 
-    /*
-      QPONMLKJIHGFEDCBA
-             QPONMLKJIH
-      GFEDCBA
-     */
-
-    /*
-      QPONMLKJIHGFEDCBA
-                  EDCBA
-             QPONM
-      LKJIHGF
-      */
-
-    do { i = ((i>>(order_nr-10)) | ( i<<10 ) ) & ((1<<order_nr)-1); }
-    while ( i >= nr ); /* this won't ever loop if nr is a power of 2 */
-
-    return i;
+  return (i & ((1 << keep) - 1)) | (low << (shift_high + keep)) | (high << keep);
 }
 
 static uint64_t tv_to_us(struct timeval *new)

@@ -1126,7 +1114,7 @@ int xc_domain_save(int xc_handle, int io
                    (batch < MAX_BATCH_SIZE) && (N < p2m_size);
                    N++ )
             {
-                int n = permute(N, p2m_size, order_nr);
+                int n = permute(N, order_nr);
 
                 if ( debug )
                 {

diff -r 76c9cf11ce23 tools/libxc/xc_domain_save.c
@@ -123,6 +123,32 @@ static inline int count_bits ( int nr, v
     for ( i = 0; i < (nr / (sizeof(unsigned long)*8)); i++, p++ )
         count += hweight32(*p);
     return count;
+}
+
+static inline int permute(unsigned long i, unsigned long nr, unsigned long order_nr)
+{
+    /* Need a simple permutation function so that we scan pages in a
+       pseudo random order, enabling us to get a better estimate of
+       the domain's page dirtying rate as we go (there are often
+       contiguous ranges of pfns that have similar behaviour, and we
+       want to mix them up. */
+  
+  unsigned char keep = 9; /* chunk of 2 MB */
+  unsigned char shift_high = (order_nr - keep) / 2;
+  unsigned char shift_low = order_nr - keep - (order_nr - keep) / 2;
+  
+  /* Check if the permutation gives an out of range number. */
+  do
+  {
+    unsigned long high = (i >> (keep + shift_low));
+    unsigned long low = (i >> keep) & ((1 << shift_low) - 1);
+    i = (i & ((1 << keep) - 1)) |
+      (low << (shift_high + keep)) | (high << keep);
+  }
+  while (i >= nr);
+
+
+  return (i);
 }
 
 static uint64_t tv_to_us(struct timeval *new)

@@ -735,6 +761,7 @@ static xen_pfn_t *map_and_save_p2m_table
         p2m_frame_list[i/FPP] = mfn_to_pfn(p2m_frame_list[i/FPP]);
     }
 
+    memset(&ctxt, 0, sizeof (ctxt));
     if ( xc_vcpu_getcontext(xc_handle, dom, 0, &ctxt.c) )
     {
         ERROR("Could not get vcpu context");

@@ -828,6 +855,8 @@ int xc_domain_save(int xc_handle, int io
 
     /* base of the region in which domain memory is mapped */
     unsigned char *region_base = NULL;
+
+    int order_nr = 0;
 
     /* bitmap of pages:
        - that should be sent this iteration (unless later marked as skip);

@@ -937,6 +966,11 @@ int xc_domain_save(int xc_handle, int io
 
     /* pretend we sent all the pages last iteration */
     sent_last_iter = p2m_size;
+
+    /* calculate the power of 2 order of p2m_size, e.g.
+       15->4 16->4 17->5 */
+    for ( i = p2m_size-1, order_nr = 0; i ; i >>= 1, order_nr++ )
+      continue;
 
     /* Setup to_send / to_fix and to_skip bitmaps */
     to_send = malloc(BITMAP_SIZE);

@@ -1088,7 +1122,7 @@ int xc_domain_save(int xc_handle, int io
                    (batch < MAX_BATCH_SIZE) && (N < p2m_size);
                    N++ )
             {
-                int n = N;
+                int n = permute(N, p2m_size, order_nr);
 
                 if ( debug )
                 {