4 files changed, 37 insertions, 1 deletions

diff --git a/Documentation/arm64/tagged-pointers.txt b/Documentation/arm64/tagged-pointers.txt
new file mode 100644
index 000000000000..264e9841563a
--- /dev/null
+++ b/Documentation/arm64/tagged-pointers.txt
@@ -0,0 +1,34 @@
+                Tagged virtual addresses in AArch64 Linux
+                =========================================
+
+Author: Will Deacon <will.deacon@arm.com>
+Date  : 12 June 2013
+
+This document briefly describes the provision of tagged virtual
+addresses in the AArch64 translation system and their potential uses
+in AArch64 Linux.
+
+The kernel configures the translation tables so that translations made
+via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
+the virtual address ignored by the translation hardware. This frees up
+this byte for application use, with the following caveats:
+
+        (1) The kernel requires that all user addresses passed to EL1
+            are tagged with tag 0x00. This means that any syscall
+            parameters containing user virtual addresses *must* have
+            their top byte cleared before trapping to the kernel.
+
+        (2) Tags are not guaranteed to be preserved when delivering
+            signals. This means that signal handlers in applications
+            making use of tags cannot rely on the tag information for
+            user virtual addresses being maintained for fields inside
+            siginfo_t. One exception to this rule is for signals raised
+            in response to debug exceptions, where the tag information
+            will be preserved.
+
+        (3) Special care should be taken when using tagged pointers,
+            since it is likely that C compilers will not hazard two
+            addresses differing only in the upper bits.
+
+The architecture prevents the use of a tagged PC, so the upper byte will
+be set to a sign-extension of bit 55 on exception return.
diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index e182a356c979..d57e66845c86 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h
@@ -122,5 +122,6 @@
 #define TCR_TG1_64K             (UL(1) << 30)
 #define TCR_IPS_40BIT           (UL(2) << 32)
 #define TCR_ASID16              (UL(1) << 36)
+#define TCR_TBI0                (UL(1) << 37)
 
 #endif
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 57640df9d70c..3881fd115ebb 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -423,6 +423,7 @@ el0_da:
          * Data abort handling
          */
         mrs     x0, far_el1
+        bic     x0, x0, #(0xff << 56)
         disable_step x1
         isb
         enable_dbg
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index f84fcf71f129..b1b31bbc967b 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -147,7 +147,7 @@ ENTRY(__cpu_setup)
          * both user and kernel.
          */
         ldr     x10, =TCR_TxSZ(VA_BITS) | TCR_FLAGS | TCR_IPS_40BIT | \
-                      TCR_ASID16 | (1 << 31)
+                      TCR_ASID16 | TCR_TBI0 | (1 << 31)
 #ifdef CONFIG_ARM64_64K_PAGES
         orr     x10, x10, TCR_TG0_64K
         orr     x10, x10, TCR_TG1_64K