[xen master] x86/mm: split unmapping and marking-as-I/O in arch_init_memory()

[patchbot@xxxxxxx]

commit 248096b113fefd36260afb06a2520781ec37f9e8
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Aug 18 10:43:28 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Aug 18 10:43:28 2025 +0200

    x86/mm: split unmapping and marking-as-I/O in arch_init_memory()
    
    The unmapping part also wants to cover UNUSABLE regions, and it will now
    be necessary for space outside the low 16Mb (wherever Xen is placed).
    
    While there, limit the scopes of involved variables.
    
    Fixes: e4dd91ea85a3 ("x86: Ensure RAM holes really are not mapped in Xen's 
ongoing 1:1 physmap")
    Fixes: 7cd7f2f5e116 ("x86/boot: Remove the preconstructed low 16M superpage 
mappings")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/mm.c | 63 +++++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 47 insertions(+), 16 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index e7fd56c7ce..9ca3469a78 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -275,8 +275,6 @@ static void __init assign_io_page(struct page_info *page)
 
 void __init arch_init_memory(void)
 {
-    unsigned long i, pfn, rstart_pfn, rend_pfn, iostart_pfn, ioend_pfn;
-
     /*
      * Basic guest-accessible flags:
      *   PRESENT, R/W, USER, A/D, AVAIL[0,1,2], AVAIL_HIGH, NX (if available).
@@ -292,12 +290,55 @@ void __init arch_init_memory(void)
      * case the low 1MB.
      */
     BUG_ON(pvh_boot && trampoline_phys != 0x1000);
-    for ( i = 0; i < 0x100; i++ )
+    for ( unsigned int i = 0; i < MB(1) >> PAGE_SHIFT; i++ )
         assign_io_page(mfn_to_page(_mfn(i)));
 
-    /* Any areas not specified as RAM by the e820 map are considered I/O. */
-    for ( i = 0, pfn = 0; pfn < max_page; i++ )
+    /*
+     * Any areas not specified as RAM by the e820 map want to have no mappings.
+     * We may have established some by mapping more than necessary in head.S,
+     * due to the use of super-pages there.
+     */
+    for ( unsigned long i = 0, pfn = 0,
+                        rlimit_pfn = PFN_DOWN(PAGE_ALIGN_2M(__pa(_end)));
+          pfn < rlimit_pfn; i++ )
     {
+        unsigned long rstart_pfn, rend_pfn, start_pfn;
+
+        while ( i < e820.nr_map &&
+                e820.map[i].type != E820_RAM )
+            i++;
+
+        if ( i >= e820.nr_map )
+        {
+            /* No more RAM regions: Unmap right to upper boundary. */
+            rstart_pfn = rend_pfn = rlimit_pfn;
+        }
+        else
+        {
+            /* Unmap just up as far as next RAM region. */
+            rstart_pfn = min(rlimit_pfn, PFN_UP(e820.map[i].addr));
+            rend_pfn   = max(rstart_pfn,
+                             PFN_DOWN(e820.map[i].addr + e820.map[i].size));
+        }
+
+        /* NB: _start is already 2Mb-aligned. */
+        start_pfn = max(pfn, PFN_DOWN(__pa(_start)));
+        if ( start_pfn < rstart_pfn )
+            destroy_xen_mappings((unsigned long)mfn_to_virt(start_pfn),
+                                 (unsigned long)mfn_to_virt(rstart_pfn));
+
+        /* Skip the RAM region. */
+        pfn = rend_pfn;
+    }
+
+    /*
+     * Any areas not specified as RAM or UNUSABLE by the e820 map are
+     * considered I/O.
+     */
+    for ( unsigned long i = 0, pfn = 0; pfn < max_page; i++ )
+    {
+        unsigned long rstart_pfn, rend_pfn;
+
         while ( (i < e820.nr_map) &&
                 (e820.map[i].type != E820_RAM) &&
                 (e820.map[i].type != E820_UNUSABLE) )
@@ -317,17 +358,6 @@ void __init arch_init_memory(void)
                                PFN_DOWN(e820.map[i].addr + e820.map[i].size));
         }
 
-        /*
-         * Make sure any Xen mappings of RAM holes above 1MB are blown away.
-         * In particular this ensures that RAM holes are respected even in
-         * the statically-initialised 1-16MB mapping area.
-         */
-        iostart_pfn = max_t(unsigned long, pfn, 1UL << (20 - PAGE_SHIFT));
-        ioend_pfn = min(rstart_pfn, 16UL << (20 - PAGE_SHIFT));
-        if ( iostart_pfn < ioend_pfn )
-            destroy_xen_mappings((unsigned long)mfn_to_virt(iostart_pfn),
-                                 (unsigned long)mfn_to_virt(ioend_pfn));
-
         /* Mark as I/O up to next RAM region. */
         for ( ; pfn < rstart_pfn; pfn++ )
         {
@@ -365,6 +395,7 @@ void __init arch_init_memory(void)
                     const l3_pgentry_t *l3idle = map_l3t_from_l4e(
                             idle_pg_table[l4_table_offset(split_va)]);
                     l3_pgentry_t *l3tab = map_domain_page(l3mfn);
+                    unsigned int i;
 
                     for ( i = 0; i < l3_table_offset(split_va); ++i )
                         l3tab[i] = l3idle[i];
--
generated by git-patchbot for /home/xen/git/xen.git#master