[RFC,v2,07/12] xen/arm64: Rework the memory layout

Commit Message

Julien Grall Oct. 22, 2022, 3:04 p.m. UTC

From: Julien Grall <[e-mail]>

Xen is currently not fully compliant with the Arm Arm because it will
switch the TTBR with the MMU on.

In order to be compliant, we need to disable the MMU before
switching the TTBR. The implication is the page-tables should
contain an identity mapping of the code switching the TTBR.

In most of the case we expect Xen to be loaded in low memory. I am aware
of one platform (i.e AMD Seattle) where the memory start above 512GB.
To give us some slack, consider that Xen may be loaded in the first 2TB
of the physical address space.

The memory layout is reshuffled to keep the first two slots of the zeroeth
level free. Xen will now be loaded at (2TB + 2MB). This requires a slight
tweak of the boot code because XEN_VIRT_START cannot be used as an
immediate.

This reshuffle will make trivial to create a 1:1 mapping when Xen is
loaded below 2TB.

Signed-off-by: Julien Grall <[e-mail]>
----

    Changes in v2:
        - Reword the commit message
        - Load Xen at 2TB + 2MB
        - Update the documentation to reflect the new layout
---
 xen/arch/arm/arm64/head.S         |  3 ++-
 xen/arch/arm/include/asm/config.h | 34 +++++++++++++++++++++----------
 xen/arch/arm/mm.c                 | 11 +++++-----
 3 files changed, 31 insertions(+), 17 deletions(-)

Patch

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index ad014716db6f..23c2c7491db2 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -607,7 +607,8 @@  create_page_tables:
          * need an additional 1:1 mapping, the virtual mapping will
          * suffice.
          */
-        cmp   x19, #XEN_VIRT_START
+        ldr   x0, =XEN_VIRT_START
+        cmp   x19, x0
         bne   1f
         ret
 1:
diff --git a/xen/arch/arm/include/asm/config.h b/xen/arch/arm/include/asm/config.h
index f381c471f67a..0ed9a0505080 100644
--- a/xen/arch/arm/include/asm/config.h
+++ b/xen/arch/arm/include/asm/config.h
@@ -72,15 +72,12 @@ 
 #include <xen/page-size.h>
 
 /*
- * Common ARM32 and ARM64 layout:
+ * ARM32 layout:
  *   0  -   2M   Unmapped
  *   2M -   4M   Xen text, data, bss
  *   4M -   6M   Fixmap: special-purpose 4K mapping slots
  *   6M -  10M   Early boot mapping of FDT
- *   10M - 12M   Livepatch vmap (if compiled in)
- *
- * ARM32 layout:
- *   0  -  12M   <COMMON>
+ *  10M -  12M   Livepatch vmap (if compiled in)
  *
  *  32M - 128M   Frametable: 24 bytes per page for 16GB of RAM
  * 256M -   1G   VMAP: ioremap and early_ioremap use this virtual address
@@ -90,8 +87,17 @@ 
  *   2G -   4G   Domheap: on-demand-mapped
  *
  * ARM64 layout:
- * 0x0000000000000000 - 0x0000007fffffffff (512GB, L0 slot [0])
- *   0  -  12M   <COMMON>
+ * 0x0000000000000000 - 0x00001fffffffffff (2TB, L0 slots [0..1])
+ *
+ *  Reserved to identity map Xen
+ *
+ * 0x0000020000000000 - 0x000028fffffffff (512TB, L0 slot [2]
+ *  (Relative offsets)
+ *   0  -   2M   Unmapped
+ *   2M -   4M   Xen text, data, bss
+ *   4M -   6M   Fixmap: special-purpose 4K mapping slots
+ *   6M -  10M   Early boot mapping of FDT
+ *  10M -  12M   Livepatch vmap (if compiled in)
  *
  *   1G -   2G   VMAP: ioremap and early_ioremap
  *
@@ -107,7 +113,17 @@ 
  *  Unused
  */
 
+#ifdef CONFIG_ARM_32
 #define XEN_VIRT_START          _AT(vaddr_t, MB(2))
+#else
+
+#define SLOT0_ENTRY_BITS  39
+#define SLOT0(slot) (_AT(vaddr_t,slot) << SLOT0_ENTRY_BITS)
+#define SLOT0_ENTRY_SIZE  SLOT0(1)
+
+#define XEN_VIRT_START          (SLOT0(2) + _AT(vaddr_t, MB(2)))
+#endif
+
 #define XEN_VIRT_SIZE           _AT(vaddr_t, MB(2))
 
 #define FIXMAP_VIRT_START       (XEN_VIRT_START + XEN_VIRT_SIZE)
@@ -164,10 +180,6 @@ 
 
 #else /* ARM_64 */
 
-#define SLOT0_ENTRY_BITS  39
-#define SLOT0(slot) (_AT(vaddr_t,slot) << SLOT0_ENTRY_BITS)
-#define SLOT0_ENTRY_SIZE  SLOT0(1)
-
 #define VMAP_VIRT_START  GB(1)
 #define VMAP_VIRT_SIZE   GB(1)
 
diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 306507d7bced..2c6648a0dfe5 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -162,7 +162,7 @@  static void __init __maybe_unused build_assertions(void)
 #endif
     /* Page table structure constraints */
 #ifdef CONFIG_ARM_64
-    BUILD_BUG_ON(zeroeth_table_offset(XEN_VIRT_START));
+    BUILD_BUG_ON(zeroeth_table_offset(XEN_VIRT_START) < 2);
 #endif
     BUILD_BUG_ON(first_table_offset(XEN_VIRT_START));
 #ifdef CONFIG_ARCH_MAP_DOMAIN_PAGE
@@ -507,10 +507,11 @@  void __init setup_pagetables(unsigned long boot_phys_offset)
     phys_offset = boot_phys_offset;
 
 #ifdef CONFIG_ARM_64
-    p = (void *) xen_pgtable;
-    p[0] = pte_of_xenaddr((uintptr_t)xen_first);
-    p[0].pt.table = 1;
-    p[0].pt.xn = 0;
+    pte = pte_of_xenaddr((uintptr_t)xen_first);
+    pte.pt.table = 1;
+    pte.pt.xn = 0;
+    xen_pgtable[zeroeth_table_offset(XEN_VIRT_START)] = pte;
+
     p = (void *) xen_first;
 #else
     p = (void *) cpu0_pgtable;