Subject: AMD/IOMMU: don't needlessly trigger errors/crashes when unmapping a page
From: Jan Beulich <[email-masked]>

Unmapping a page which has never been mapped should be a no-op (note how
it already is in case there was no root page table allocated). There's
in particular no need to grow the number of page table levels in use,
and there's also no need to allocate intermediate page tables except
when needing to split a large page.

Reported-by: XXX PERSON <XXX EMAIL>3
Signed-off-by: Andrew Cooper <[email-masked]>
Signed-off-by: Jan Beulich <[email-masked]>
Reviewed-by: Paul Durrant <[email-masked]>
Acked-by: Andrew Cooper <[email-masked]>

--- a/xen/drivers/passthrough/amd/iommu_map.c
+++ b/xen/drivers/passthrough/amd/iommu_map.c
@@ -456,7 +456,7 @@ static int iommu_merge_pages(struct doma
  * page tables.
  */
 static int iommu_pde_from_gfn(struct domain *d, unsigned long pfn, 
-                              unsigned long pt_mfn[])
+                              unsigned long pt_mfn[], bool map)
 {
     u64 *pde, *next_table_vaddr;
     unsigned long  next_table_mfn;
@@ -470,6 +470,13 @@ static int iommu_pde_from_gfn(struct dom
     BUG_ON( table == NULL || level < IOMMU_PAGING_MODE_LEVEL_1 || 
             level > IOMMU_PAGING_MODE_LEVEL_6 );
 
+    /*
+     * A frame number past what the current page tables can represent can't
+     * possibly have a mapping.
+     */
+    if ( pfn >> (PTE_PER_TABLE_SHIFT * level) )
+        return 0;
+
     next_table_mfn = page_to_mfn(table);
 
     if ( level == IOMMU_PAGING_MODE_LEVEL_1 )
@@ -530,6 +537,9 @@ static int iommu_pde_from_gfn(struct dom
         /* Install lower level page table for non-present entries */
         else if ( !iommu_is_pte_present((u32*)pde) )
         {
+            if ( !map )
+                return 0;
+
             if ( next_table_mfn == 0 )
             {
                 table = alloc_amd_iommu_pgtable();
@@ -688,7 +698,7 @@ int amd_iommu_map_page(struct domain *d,
         }
     }
 
-    if ( iommu_pde_from_gfn(d, gfn, pt_mfn) || (pt_mfn[1] == 0) )
+    if ( iommu_pde_from_gfn(d, gfn, pt_mfn, true) || (pt_mfn[1] == 0) )
     {
         spin_unlock(&hd->arch.mapping_lock);
         AMD_IOMMU_DEBUG("Invalid IO pagetable entry gfn = %lx\n", gfn);
@@ -767,23 +777,7 @@ int amd_iommu_unmap_page(struct domain *
         return 0;
     }
 
-    /* Since HVM domain is initialized with 2 level IO page table,
-     * we might need a deeper page table for lager gfn now */
-    if ( is_hvm_domain(d) )
-    {
-        int rc = update_paging_mode(d, gfn);
-
-        if ( rc )
-        {
-            spin_unlock(&hd->arch.mapping_lock);
-            AMD_IOMMU_DEBUG("Update page mode failed gfn = %lx\n", gfn);
-            if ( rc != -EADDRNOTAVAIL )
-                domain_crash(d);
-            return rc;
-        }
-    }
-
-    if ( iommu_pde_from_gfn(d, gfn, pt_mfn) || (pt_mfn[1] == 0) )
+    if ( iommu_pde_from_gfn(d, gfn, pt_mfn, false) )
     {
         spin_unlock(&hd->arch.mapping_lock);
         AMD_IOMMU_DEBUG("Invalid IO pagetable entry gfn = %lx\n", gfn);
@@ -791,8 +785,11 @@ int amd_iommu_unmap_page(struct domain *
         return -EFAULT;
     }
 
-    /* mark PTE as 'page not present' */
-    clear_iommu_pte_present(pt_mfn[1], gfn);
+    if ( pt_mfn[1] )
+    {
+        /* Mark PTE as 'page not present'. */
+        clear_iommu_pte_present(pt_mfn[1], gfn);
+    }
 
     /* No further merging in amd_iommu_map_page(), as the logic doesn't cope. */
     hd->arch.no_merge = true;