ARM: 6396/1: Add SWP/SWPB emulation for ARMv7 processors

The SWP instruction was deprecated in the ARMv6 architecture,
superseded by the LDREX/STREX family of instructions for
load-linked/store-conditional operations. The ARMv7 multiprocessing
extensions mandate that SWP/SWPB instructions are treated as undefined
from reset, with the ability to enable them through the System Control
Register SW bit.

This patch adds the alternative solution to emulate the SWP and SWPB
instructions using LDREX/STREX sequences, and log statistics to
/proc/cpu/swp_emulation. To correctly deal with copy-on-write, it also
modifies cpu_v7_set_pte_ext to change the mappings to priviliged RO when
user RO.

Signed-off-by: Leif Lindholm <leif.lindholm@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

2 files changed, 31 insertions, 0 deletions

diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index 6d05f79a8cd2..8493ed04797a 100644
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -636,6 +636,33 @@ config ARM_THUMBEE
           Say Y here if you have a CPU with the ThumbEE extension and code to
           make use of it. Say N for code that can run on CPUs without ThumbEE.
 
+config SWP_EMULATE
+        bool "Emulate SWP/SWPB instructions"
+        depends on CPU_V7
+        select HAVE_PROC_CPU if PROC_FS
+        default y if SMP
+        help
+          ARMv6 architecture deprecates use of the SWP/SWPB instructions.
+          ARMv7 multiprocessing extensions introduce the ability to disable
+          these instructions, triggering an undefined instruction exception
+          when executed. Say Y here to enable software emulation of these
+          instructions for userspace (not kernel) using LDREX/STREX.
+          Also creates /proc/cpu/swp_emulation for statistics.
+
+          In some older versions of glibc [<=2.8] SWP is used during futex
+          trylock() operations with the assumption that the code will not
+          be preempted. This invalid assumption may be more likely to fail
+          with SWP emulation enabled, leading to deadlock of the user
+          application.
+
+          NOTE: when accessing uncached shared regions, LDREX/STREX rely
+          on an external transaction monitoring block called a global
+          monitor to maintain update atomicity. If your system does not
+          implement a global monitor, this option can cause programs that
+          perform SWP operations to uncached memory to deadlock.
+
+          If unsure, say Y.
+
 config CPU_BIG_ENDIAN
         bool "Build big-endian kernel"
         depends on ARCH_SUPPORTS_BIG_ENDIAN
diff --git a/arch/arm/mm/proc-v7.S b/arch/arm/mm/proc-v7.S
index cfc11afab1fb..2b5b20baf80d 100644
--- a/arch/arm/mm/proc-v7.S
+++ b/arch/arm/mm/proc-v7.S
@@ -314,6 +314,10 @@ __v7_setup:
 #ifdef CONFIG_CPU_ENDIAN_BE8
         orr     r6, r6, #1 << 25                @ big-endian page tables
 #endif
+#ifdef CONFIG_SWP_EMULATE
+        orr     r5, r5, #(1 << 10)              @ set SW bit in "clear"
+        bic     r6, r6, #(1 << 10)              @ clear it in "mmuset"
+#endif
         mrc     p15, 0, r0, c1, c0, 0           @ read control register
         bic     r0, r0, r5                      @ clear bits them
         orr     r0, r0, r6                      @ set them