[Xen-devel] [PATCH v2 4/4] xen: arm: avoid unnecessary additional mappings 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.

For example consider an arm32 system (for simplicity) with Xen loaded at
address 0x40402000. As a virtual address this corresponds to walking offsets 1,
2 and 2 at the first, second and third levels respectively.

When creating the identity map we will therefore create a 1GB super mapping at
0x40000000 for the identity map, which is the one we want to use.

However when considering the second level we will see the offset 2 and create a
2MB mapping in slot 2 of boot_second. Since boot_second is mapped in slot 0 of
boot_first this corresponds to an unwanted mapping from virtual address
0x00400000 to physical address 0x40400000.

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

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
---
v2: Expanded on commit message.
    Handle/accept being loaded at exactly 2M
---
 xen/arch/arm/arm32/head.S |   20 +++++++++++++++++---
 xen/arch/arm/arm64/head.S |   19 ++++++++++++++++---
 2 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
index 0a76c2e..caf7934 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 +250,14 @@ cpu_init_done:
          * mapping. So each CPU must rebuild the page tables here with
          * the 1:1 in place. */
 
+        /* If Xen is loaded at exactly XEN_VIRT_START then we don't
+         * need an additional 1:1 mapping, the virtual mapping will
+         * suffice.
+         */
+        cmp   r9, #XEN_VIRT_START
+        moveq r6, #1                 /* r6 := identity map now in place */
+        movne 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 +283,7 @@ 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 +299,8 @@ 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
         lsr   r2, r9, #SECOND_SHIFT  /* Offset of base paddr in boot_second */
         mov   r3, #0x0ff             /* r2 := LPAE entries mask */
         orr   r3, r3, #0x100
@@ -303,6 +314,7 @@ cpu_init_done:
         mov   r3, #0x0
         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
@@ -327,8 +339,10 @@ cpu_init_done:
 
         /* boot pagetable setup complete */
 
-        b     1f
-
+        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
 virtphys_clash:
         /* Identity map clashes with boot_third, which we cannot handle yet */
         PRINT("Unable to build boot page tables - virt and phys addresses 
clash.\r\n")
diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 6ba9d94..f28b74a 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 +253,13 @@ skip_bss:
          * mapping. So each CPU must rebuild the page tables here with
          * the 1:1 in place. */
 
+        /* If Xen is loaded at exactly XEN_VIRT_START then we don't
+         * need an additional 1:1 mapping, the virtual mapping will
+         * suffice.
+         */
+        cmp   x19, #XEN_VIRT_START
+        cset  x25, eq                /* x25 := identity map in place, or not */
+
         /* Write Xen's PT's paddr into TTBR0_EL2 */
         ldr   x4, =boot_pgtable
         add   x4, x4, x20            /* x4 := paddr (boot_pagetable) */
@@ -293,6 +300,7 @@ skip_bss:
         and   x1, x1, #0x1ff         /* x1 := Slot offset */
         lsl   x1, x1, #3
         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 +314,7 @@ 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 +324,7 @@ 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 +338,7 @@ 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
@@ -338,6 +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
@@ -361,8 +373,9 @@ skip_bss:
 
         /* boot pagetable setup complete */
 
-        b     1f
-
+        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
 virtphys_clash:
         /* Identity map clashes with boot_third, which we cannot handle yet */
         PRINT("Unable to build boot page tables - virt and phys addresses 
clash.\r\n")
-- 
1.7.10.4


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