[Xen-devel] [PATCH 3/6] x86/boot: Remove the preconstructed low 16M superpage mappings

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

First, it is undefined to have superpages and MTRRs disagree on cacheability
boundaries, and nothing this early in boot has checked that it is safe to use
superpages here.

Furthermore, nothing actually uses the mappings on boot.  Build these entries
in the directmap when walking the E820 table along with everything else.

As a consequence, there are now no _PAGE_PRESENT entries between
__page_tables_{start,end} which need to skip relocation.  This simplifies the
MB1/2 entry path logic to remove the l2_identmap[] special case.

The low 2M (using 4k pages) is retained for now.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/boot/head.S          | 10 ++--------
 xen/arch/x86/boot/x86_64.S        | 17 ++++++-----------
 xen/arch/x86/setup.c              | 12 ++++++------
 xen/arch/x86/x86_64/asm-offsets.c |  3 ---
 4 files changed, 14 insertions(+), 28 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 8d0ffbd1b0..7ee4511e26 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -661,15 +661,9 @@ trampoline_setup:
         mov     %eax,sym_fs(boot_tsc_stamp)
         mov     %edx,sym_fs(boot_tsc_stamp)+4
 
-        /*
-         * Update frame addresses in page tables excluding l2_identmap
-         * without its first entry which points to l1_identmap.
-         */
+        /* Relocate pagetables to point at Xen's current location in memory. */
         mov     $((__page_tables_end-__page_tables_start)/8),%ecx
-        mov     $(((l2_identmap-__page_tables_start)/8)+1),%edx
-1:      cmp     $((l2_identmap+l2_identmap_sizeof-__page_tables_start)/8),%ecx
-        cmove   %edx,%ecx
-        testl   $_PAGE_PRESENT,sym_fs(__page_tables_start)-8(,%ecx,8)
+1:      testl   $_PAGE_PRESENT,sym_fs(__page_tables_start)-8(,%ecx,8)
         jz      2f
         add     %esi,sym_fs(__page_tables_start)-8(,%ecx,8)
 2:      loop    1b
diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index b54d3aceea..30c82f9d5c 100644
--- a/xen/arch/x86/boot/x86_64.S
+++ b/xen/arch/x86/boot/x86_64.S
@@ -66,24 +66,19 @@ l1_identmap:
         .size l1_identmap, . - l1_identmap
 
 /*
- * __page_tables_start does not cover l1_identmap because it (l1_identmap)
- * contains 1-1 mappings. This means that frame addresses of these mappings
- * are static and should not be updated at runtime.
+ * __page_tables_{start,end} cover the range of pagetables which need
+ * relocating as Xen moves around physical memory.  i.e. each sym_offs()
+ * reference to a different pagetable in the Xen image.
  */
 GLOBAL(__page_tables_start)
 
 /*
- * Space for mapping the first 4GB of memory, with the first 16 megabytes
- * actualy mapped (mostly using superpages).  Uses 4x 4k pages.
+ * Space for 4G worth of 2M mappings, first 2M actually mapped via
+ * l1_identmap[].  Uses 4x 4k pages.
  */
 GLOBAL(l2_identmap)
         .quad sym_offs(l1_identmap) + __PAGE_HYPERVISOR
-        idx = 1
-        .rept 7
-        .quad (idx << L2_PAGETABLE_SHIFT) | PAGE_HYPERVISOR | _PAGE_PSE
-        idx = idx + 1
-        .endr
-        .fill 4 * L2_PAGETABLE_ENTRIES - 8, 8, 0
+        .fill 4 * L2_PAGETABLE_ENTRIES - 1, 8, 0
         .size l2_identmap, . - l2_identmap
 
 /*
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index ed54f79fea..452f5bdd37 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1020,8 +1020,8 @@ void __init noreturn __start_xen(unsigned long mbi_p)
      *
      * We require superpage alignment because the boot allocator is
      * not yet initialised. Hence we can only map superpages in the
-     * address range BOOTSTRAP_MAP_BASE to 4GB, as this is guaranteed
-     * not to require dynamic allocation of pagetables.
+     * address range 2MB to 4GB, as this is guaranteed not to require
+     * dynamic allocation of pagetables.
      *
      * As well as mapping superpages in that range, in preparation for
      * initialising the boot allocator, we also look for a region to which
@@ -1036,10 +1036,10 @@ void __init noreturn __start_xen(unsigned long mbi_p)
         if ( boot_e820.map[i].type != E820_RAM )
             continue;
 
-        /* Superpage-aligned chunks from BOOTSTRAP_MAP_BASE. */
+        /* Superpage-aligned chunks from 2MB. */
         s = (boot_e820.map[i].addr + mask) & ~mask;
         e = (boot_e820.map[i].addr + boot_e820.map[i].size) & ~mask;
-        s = max_t(uint64_t, s, BOOTSTRAP_MAP_BASE);
+        s = max_t(uint64_t, s, MB(2));
         if ( s >= e )
             continue;
 
@@ -1346,8 +1346,8 @@ void __init noreturn __start_xen(unsigned long mbi_p)
 
         set_pdx_range(s >> PAGE_SHIFT, e >> PAGE_SHIFT);
 
-        /* Need to create mappings above BOOTSTRAP_MAP_BASE. */
-        map_s = max_t(uint64_t, s, BOOTSTRAP_MAP_BASE);
+        /* Need to create mappings above 2MB. */
+        map_s = max_t(uint64_t, s, MB(2));
         map_e = min_t(uint64_t, e,
                       ARRAY_SIZE(l2_identmap) << L2_PAGETABLE_SHIFT);
 
diff --git a/xen/arch/x86/x86_64/asm-offsets.c 
b/xen/arch/x86/x86_64/asm-offsets.c
index f9cb78cfdb..07d2155bf5 100644
--- a/xen/arch/x86/x86_64/asm-offsets.c
+++ b/xen/arch/x86/x86_64/asm-offsets.c
@@ -165,8 +165,5 @@ void __dummy__(void)
     OFFSET(MB2_efi64_ih, multiboot2_tag_efi64_ih_t, pointer);
     BLANK();
 
-    DEFINE(l2_identmap_sizeof, sizeof(l2_identmap));
-    BLANK();
-
     OFFSET(DOMAIN_vm_assist, struct domain, vm_assist);
 }
-- 
2.11.0


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