[Xen-devel] [PATCH] minios: don't rely on specific page table allocation scheme

[Juergen Gross <jgross@xxxxxxxx>]

Today mini-os is making assumptions how the page tables it is started
with are being allocated. Especially it is using the number of page
table frames to calculate which is the first unmapped pfn.

Instead of relying on page table number assumptions just look into the
page tables to find the first pfn not already mapped.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 arch/x86/mm.c         | 47 ++++++++++++++++++++++++++++++++++++-----------
 include/x86/arch_mm.h |  7 -------
 2 files changed, 36 insertions(+), 18 deletions(-)

diff --git a/arch/x86/mm.c b/arch/x86/mm.c
index 9c6d1b8..5d7c006 100644
--- a/arch/x86/mm.c
+++ b/arch/x86/mm.c
@@ -200,8 +200,8 @@ static void build_pagetable(unsigned long *start_pfn, 
unsigned long *max_pfn)
     int count = 0;
     int rc;
 
-    pfn_to_map = 
-        (start_info.nr_pt_frames - NOT_L1_FRAMES) * L1_PAGETABLE_ENTRIES;
+    pfn_to_map = (*start_pfn + L1_PAGETABLE_ENTRIES - 1) &
+                 ~(L1_PAGETABLE_ENTRIES - 1);
 
     if ( *max_pfn >= virt_to_pfn(HYPERVISOR_VIRT_START) )
     {
@@ -229,9 +229,15 @@ static void build_pagetable(unsigned long *start_pfn, 
unsigned long *max_pfn)
 #if defined(__x86_64__)
         offset = l4_table_offset(start_address);
         /* Need new L3 pt frame */
-        if ( !(start_address & L3_MASK) )
-            if ( need_pt_frame(start_address, L3_FRAME) ) 
-                new_pt_frame(&pt_pfn, pt_mfn, offset, L3_FRAME);
+        if ( !(tab[offset] & _PAGE_PRESENT) )
+        {
+            if ( !need_pt_frame(start_address, L3_FRAME) )
+            {
+                printk("ERROR: build_pagetable(): L3 frame not present\n");
+                do_exit();
+            }
+            new_pt_frame(&pt_pfn, pt_mfn, offset, L3_FRAME);
+        }
 
         page = tab[offset];
         pt_mfn = pte_to_mfn(page);
@@ -239,18 +245,37 @@ static void build_pagetable(unsigned long *start_pfn, 
unsigned long *max_pfn)
 #endif
         offset = l3_table_offset(start_address);
         /* Need new L2 pt frame */
-        if ( !(start_address & L2_MASK) )
-            if ( need_pt_frame(start_address, L2_FRAME) )
-                new_pt_frame(&pt_pfn, pt_mfn, offset, L2_FRAME);
+        if ( !(tab[offset] & _PAGE_PRESENT) )
+        {
+            if ( !need_pt_frame(start_address, L2_FRAME) )
+            {
+                printk("ERROR: build_pagetable(): L2 frame not present\n");
+                do_exit();
+            }
+            new_pt_frame(&pt_pfn, pt_mfn, offset, L2_FRAME);
+        }
 
         page = tab[offset];
         pt_mfn = pte_to_mfn(page);
         tab = to_virt(mfn_to_pfn(pt_mfn) << PAGE_SHIFT);
         offset = l2_table_offset(start_address);        
         /* Need new L1 pt frame */
-        if ( !(start_address & L1_MASK) )
-            if ( need_pt_frame(start_address, L1_FRAME) )
-                new_pt_frame(&pt_pfn, pt_mfn, offset, L1_FRAME);
+        if ( !(tab[offset] & _PAGE_PRESENT) )
+        {
+            if ( !need_pt_frame(start_address, L1_FRAME) )
+            {
+                printk("ERROR: build_pagetable(): L1 frame not present\n");
+                do_exit();
+            }
+            new_pt_frame(&pt_pfn, pt_mfn, offset, L1_FRAME);
+        }
+        else if ( !(start_address & L1_MASK) )
+        {
+            /* Already mapped, skip this L1 entry. */
+            start_address += L1_PAGETABLE_ENTRIES << PAGE_SHIFT;
+            pfn_to_map += L1_PAGETABLE_ENTRIES;
+            continue;
+        }
 
         page = tab[offset];
         pt_mfn = pte_to_mfn(page);
diff --git a/include/x86/arch_mm.h b/include/x86/arch_mm.h
index 23cfca7..58f29fc 100644
--- a/include/x86/arch_mm.h
+++ b/include/x86/arch_mm.h
@@ -56,12 +56,6 @@
 
 #define L2_MASK  ((1UL << L3_PAGETABLE_SHIFT) - 1)
 
-/*
- * If starting from virtual address greater than 0xc0000000,
- * this value will be 2 to account for final mid-level page
- * directory which is always mapped in at this location.
- */
-#define NOT_L1_FRAMES           3
 #define PRIpte "016llx"
 #ifndef __ASSEMBLY__
 typedef uint64_t pgentry_t;
@@ -87,7 +81,6 @@ typedef uint64_t pgentry_t;
 #define L2_MASK  ((1UL << L3_PAGETABLE_SHIFT) - 1)
 #define L3_MASK  ((1UL << L4_PAGETABLE_SHIFT) - 1)
 
-#define NOT_L1_FRAMES           3
 #define PRIpte "016lx"
 #ifndef __ASSEMBLY__
 typedef unsigned long pgentry_t;
-- 
2.6.2


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