[PATCH 1/4] x86/PVH: de-duplicate mappings for first Mb of Dom0 memory

[Jan Beulich <jbeulich@xxxxxxxx>]

One of the changes comprising the fixes for XSA-378 disallows replacing
MMIO mappings by unintended (for this purpose) code paths. This means we
need to be more careful about the mappings put in place in this range -
mappings should be created exactly once:
- iommu_hwdom_init() comes first; it should avoid the first Mb,
- pvh_populate_p2m() should insert identity mappings only into ranges
  not populated as RAM,
- pvh_setup_acpi() should again avoid the first Mb, which was already
  dealt with at that point.

Fixes: 753cb68e6530 ("x86/p2m: guard (in particular) identity mapping entries")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Note: Conflicts with the previously submitted "IOMMU/x86: perform PV
      Dom0 mappings in batches".

--- a/xen/arch/x86/hvm/dom0_build.c
+++ b/xen/arch/x86/hvm/dom0_build.c
@@ -430,17 +430,6 @@ static int __init pvh_populate_p2m(struc
     int rc;
 #define MB1_PAGES PFN_DOWN(MB(1))
 
-    /*
-     * Memory below 1MB is identity mapped initially. RAM regions are
-     * populated and copied below, replacing the respective mappings.
-     */
-    rc = modify_identity_mmio(d, 0, MB1_PAGES, true);
-    if ( rc )
-    {
-        printk("Failed to identity map low 1MB: %d\n", rc);
-        return rc;
-    }
-
     /* Populate memory map. */
     for ( i = 0; i < d->arch.nr_e820; i++ )
     {
@@ -472,6 +461,23 @@ static int __init pvh_populate_p2m(struc
         }
     }
 
+    /* Non-RAM regions of space below 1MB get identity mapped. */
+    for ( i = rc = 0; i < MB1_PAGES; ++i )
+    {
+        p2m_type_t p2mt;
+
+        if ( mfn_eq(get_gfn_query(d, i, &p2mt), INVALID_MFN) )
+            rc = set_mmio_p2m_entry(d, _gfn(i), _mfn(i), PAGE_ORDER_4K);
+        else
+            ASSERT(p2mt == p2m_ram_rw);
+        put_gfn(d, i);
+        if ( rc )
+        {
+            printk("Failed to identity map PFN %x: %d\n", i, rc);
+            return rc;
+        }
+    }
+
     if ( cpu_has_vmx && paging_mode_hap(d) && !vmx_unrestricted_guest(v) )
     {
         /*
@@ -1095,6 +1101,17 @@ static int __init pvh_setup_acpi(struct
         nr_pages = PFN_UP((d->arch.e820[i].addr & ~PAGE_MASK) +
                           d->arch.e820[i].size);
 
+        /* Memory below 1MB has been dealt with by pvh_populate_p2m(). */
+        if ( pfn < PFN_DOWN(MB(1)) )
+        {
+            if ( pfn + nr_pages <= PFN_DOWN(MB(1)) )
+                continue;
+
+            /* This shouldn't happen, but is easy to deal with. */
+            nr_pages -= PFN_DOWN(MB(1)) - pfn;
+            pfn = PFN_DOWN(MB(1));
+        }
+
         rc = modify_identity_mmio(d, pfn, nr_pages, true);
         if ( rc )
         {
--- a/xen/drivers/passthrough/x86/iommu.c
+++ b/xen/drivers/passthrough/x86/iommu.c
@@ -337,7 +337,13 @@ void __hwdom_init arch_iommu_hwdom_init(
     max_pfn = (GB(4) >> PAGE_SHIFT) - 1;
     top = max(max_pdx, pfn_to_pdx(max_pfn) + 1);
 
-    for ( i = 0; i < top; i++ )
+    /*
+     * First Mb will get mapped in one go by pvh_populate_p2m(). Avoid
+     * setting up potentially conflicting mappings here.
+     */
+    i = paging_mode_translate(d) ? PFN_DOWN(MB(1)) : 0;
+
+    for ( ; i < top; i++ )
     {
         unsigned long pfn = pdx_to_pfn(i);
         int rc;