[PATCH 1/2] x86/mm: avoid phys_to_nid() calls for invalid addresses

[Jan Beulich <jbeulich@xxxxxxxx>]

With phys_to_nid() now actively checking that a valid node ID is on
record, the two uses in paging_init() can actually trigger at least the
2nd of the assertions there. They're used to calculate allocation flags,
but the calculated flags wouldn't be used when dealing with an invalid
(unpopulated) address range. Defer the calculations such that they can
be done with a validated MFN in hands. This also does away with the
artificial calculations of an address to pass to phys_to_nid().

Note that while the variable is provably written before use, at least
some compiler versions can't actually verify that. Hence the variable
also needs to gain a (dead) initializer.

Fixes: e9c72d524fbd ("xen/x86: Use ASSERT instead of VIRTUAL_BUG_ON for 
phys_to_nid")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
RFC: With small enough a NUMA hash shift it would still be possible to
     hit an SRAT hole, despite mfn_valid() passing. Hence, like was the
     original plan, it may still be necessary to relax the checking in
     phys_to_nid() (or its designated replacements). At which point the
     value of this change here would shrink to merely reducing the
     chance of unintentionally doing NUMA_NO_NODE allocations.

--- a/xen/arch/x86/x86_64/mm.c
+++ b/xen/arch/x86/x86_64/mm.c
@@ -498,7 +498,7 @@ error:
 void __init paging_init(void)
 {
     unsigned long i, mpt_size, va;
-    unsigned int n, memflags;
+    unsigned int n, memflags = 0;
     l3_pgentry_t *l3_ro_mpt;
     l2_pgentry_t *pl2e = NULL, *l2_ro_mpt = NULL;
     struct page_info *l1_pg;
@@ -547,8 +547,6 @@ void __init paging_init(void)
     {
         BUILD_BUG_ON(RO_MPT_VIRT_START & ((1UL << L3_PAGETABLE_SHIFT) - 1));
         va = RO_MPT_VIRT_START + (i << L2_PAGETABLE_SHIFT);
-        memflags = MEMF_node(phys_to_nid(i <<
-            (L2_PAGETABLE_SHIFT - 3 + PAGE_SHIFT)));
 
         if ( cpu_has_page1gb &&
              !((unsigned long)pl2e & ~PAGE_MASK) &&
@@ -559,10 +557,15 @@ void __init paging_init(void)
             for ( holes = k = 0; k < 1 << PAGETABLE_ORDER; ++k)
             {
                 for ( n = 0; n < CNT; ++n)
-                    if ( mfn_valid(_mfn(MFN(i + k) + n * PDX_GROUP_COUNT)) )
+                {
+                    mfn = _mfn(MFN(i + k) + n * PDX_GROUP_COUNT);
+                    if ( mfn_valid(mfn) )
                         break;
+                }
                 if ( n == CNT )
                     ++holes;
+                else if ( k == holes )
+                    memflags = MEMF_node(phys_to_nid(mfn_to_maddr(mfn)));
             }
             if ( k == holes )
             {
@@ -593,8 +596,14 @@ void __init paging_init(void)
         }
 
         for ( n = 0; n < CNT; ++n)
-            if ( mfn_valid(_mfn(MFN(i) + n * PDX_GROUP_COUNT)) )
+        {
+            mfn = _mfn(MFN(i) + n * PDX_GROUP_COUNT);
+            if ( mfn_valid(mfn) )
+            {
+                memflags = MEMF_node(phys_to_nid(mfn_to_maddr(mfn)));
                 break;
+            }
+        }
         if ( n == CNT )
             l1_pg = NULL;
         else if ( (l1_pg = alloc_domheap_pages(NULL, PAGETABLE_ORDER,
@@ -663,15 +672,19 @@ void __init paging_init(void)
                  sizeof(*compat_machine_to_phys_mapping));
     for ( i = 0; i < (mpt_size >> L2_PAGETABLE_SHIFT); i++, pl2e++ )
     {
-        memflags = MEMF_node(phys_to_nid(i <<
-            (L2_PAGETABLE_SHIFT - 2 + PAGE_SHIFT)));
         for ( n = 0; n < CNT; ++n)
-            if ( mfn_valid(_mfn(MFN(i) + n * PDX_GROUP_COUNT)) )
+        {
+            mfn = _mfn(MFN(i) + n * PDX_GROUP_COUNT);
+            if ( mfn_valid(mfn) )
+            {
+                memflags = MEMF_node(phys_to_nid(mfn_to_maddr(mfn)));
                 break;
+            }
+        }
         if ( n == CNT )
             continue;
         if ( (l1_pg = alloc_domheap_pages(NULL, PAGETABLE_ORDER,
-                                               memflags)) == NULL )
+                                          memflags)) == NULL )
             goto nomem;
         map_pages_to_xen(
             RDWR_COMPAT_MPT_VIRT_START + (i << L2_PAGETABLE_SHIFT),