[Xen-devel] [PATCH 4/5] xen: arm: avoid unnecessary aliasing in boot page tables.

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

If the identity map is created at one level then avoid creating
entries further down the boot page tables, since these will be aliases at 
strange virtual address

i.e. if the load address is such that both first_table_offset and
second_table_offset are non-zero then we should avoid creating a mapping in
boot_second, because the proper mapping is already in boot_first.

We still do not handle the case where the load address is within the 2MB range
starting at XEN_VIRT_START. This is not a regression but this patch tries to
provide a more useful diagnostic message.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
---
 xen/arch/arm/arm32/head.S |   14 +++++++++++++-
 xen/arch/arm/arm64/head.S |   14 +++++++++++++-
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
index 3a72195..348ca7d 100644
--- a/xen/arch/arm/arm32/head.S
+++ b/xen/arch/arm/arm32/head.S
@@ -45,7 +45,7 @@
  *   r3  -
  *   r4  -
  *   r5  -
- *   r6  -
+ *   r6  - identity map in place
  *   r7  - CPUID
  *   r8  - DTB address (boot CPU only)
  *   r9  - paddr(start)
@@ -250,6 +250,8 @@ cpu_init_done:
          * mapping. So each CPU must rebuild the page tables here with
          * the 1:1 in place. */
 
+        mov   r6, #0                /* r6 := identity map not yet in place */
+
         /* Write Xen's PT's paddr into the HTTBR */
         ldr   r4, =boot_pgtable
         add   r4, r4, r10            /* r4 := paddr (boot_pagetable) */
@@ -275,6 +277,7 @@ cpu_init_done:
         orr   r2, r2, #PT_LOWER(MEM)
         lsl   r1, r1, #3             /* r1 := Slot offset */
         strd  r2, r3, [r4, r1]       /* Mapping of paddr(start) */
+        mov   r6, #1                 /* r6 := identity map now in place */
 
 1:      /* Setup boot_second: */
         ldr   r4, =boot_second
@@ -290,6 +293,8 @@ cpu_init_done:
         strd  r2, r3, [r4, #8]       /* Map it in slot 1 */
 
         /* ... map of paddr(start) in boot_second */
+        cmp   r6, #1                 /* r6 is set if already created */
+        beq   1f
         lsrs  r1, r9, #SECOND_SHIFT  /* Offset of base paddr in boot_second */
         mov   r2, #0x0ff             /* r2 := LPAE entries mask */
         orr   r2, r2, #0x100
@@ -307,6 +312,7 @@ cpu_init_done:
         orr   r2, r2, #PT_LOWER(MEM)
         lsl   r1, r1, #3             /* r1 := Slot offset */
         strd  r2, r3, [r4, r1]       /* Mapping of paddr(start) */
+        mov   r6, #1                 /* r6 := identity map now in place */
 
         /* Setup boot_third: */
 1:      ldr   r4, =boot_third
@@ -330,6 +336,12 @@ cpu_init_done:
 
         /* boot pagetable setup complete */
 
+        cmp   r6, #1                /* Did we manage to create an identity 
mapping ? */
+        beq   1f
+        PRINT("Unable to build boot page tables - Failed to identity map 
Xen.\r\n")
+        b     fail
+
+1:
         PRINT("- Turning on paging -\r\n")
 
         ldr   r1, =paging            /* Explicit vaddr, not RIP-relative */
diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 023a9b9..7152d9c 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -62,7 +62,7 @@
  *  x22 - is_secondary_cpu
  *  x23 - UART address
  *  x24 - cpuid
- *  x25 -
+ *  x25 - identity map in place
  *  x26 -
  *  x27 -
  *  x28 -
@@ -253,6 +253,8 @@ skip_bss:
          * mapping. So each CPU must rebuild the page tables here with
          * the 1:1 in place. */
 
+        mov   x25, #0                /* x25 := identity map not yet in place */
+
         /* Write Xen's PT's paddr into TTBR0_EL2 */
         ldr   x4, =boot_pgtable
         add   x4, x4, x20            /* x4 := paddr (boot_pagetable) */
@@ -293,6 +295,7 @@ skip_bss:
 
         lsl   x1, x1, #3             /* x1 := Slot offset */
         str   x2, [x4, x1]           /* Mapping of paddr(start)*/
+        mov   x25, #1                /* x25 := identity map now in place */
 
 1:      /* Setup boot_first: */
         ldr   x4, =boot_first        /* Next level into boot_first */
@@ -306,6 +309,7 @@ skip_bss:
         str   x2, [x4, #0]           /* Map it in slot 0 */
 
         /* ... map of paddr(start) in boot_first */
+        cbnz  x25, 1f                /* x25 is set if already created */
         lsr   x2, x19, #FIRST_SHIFT  /* x2 := Offset of base paddr in 
boot_first */
         and   x1, x2, 0x1ff          /* x1 := Slot to use */
         cbz   x1, 1f                 /* It's in slot 0, map in boot_second */
@@ -315,6 +319,7 @@ skip_bss:
         orr   x2, x2, x3
         lsl   x1, x1, #3             /* x1 := Slot offset */
         str   x2, [x4, x1]           /* Create mapping of paddr(start)*/
+        mov   x25, #1                /* x25 := identity map now in place */
 
 1:      /* Setup boot_second: */
         ldr   x4, =boot_second       /* Next level into boot_second */
@@ -328,6 +333,7 @@ skip_bss:
         str   x2, [x4, #8]           /* Map it in slot 1 */
 
         /* ... map of paddr(start) in boot_second */
+        cbnz  x25, 1f                /* x25 is set if already created */
         lsr   x2, x19, #SECOND_SHIFT /* x2 := Offset of base paddr in 
boot_second */
         and   x1, x2, 0x1ff          /* x1 := Slot to use */
         cmp   x1, #1
@@ -343,6 +349,7 @@ skip_bss:
         orr   x2, x2, x3
         lsl   x1, x1, #3             /* x1 := Slot offset */
         str   x2, [x4, x1]           /* Create mapping of paddr(start)*/
+        mov   x25, #1                /* x25 := identity map now in place */
 
 1:      /* Setup boot_third: */
         ldr   x4, =boot_third
@@ -366,6 +373,11 @@ skip_bss:
 
         /* boot pagetable setup complete */
 
+        cbnz  x25, 1f                /* Did we manage to create an identity 
mapping ? */
+        PRINT("Unable to build boot page tables - Failed to identity map 
Xen.\r\n")
+        b     fail
+
+1:
         PRINT("- Turning on paging -\r\n")
 
         ldr   x1, =paging            /* Explicit vaddr, not RIP-relative */
-- 
1.7.10.4


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