[lttng-dev] [PATCH urcu 3/4] Use DMB only on ARMv7

[Michael Jeanson]

Remove the configure time CONFIG_RCU_ARM_HAVE_DMB option and replace it
by compile time detection based on the ARM ISA version. This makes sure
we unconditionnaly use the DMB instruction only on ARMv7 where it's part
of the baseline ISA.

This will change the behavior on ARMv6 platform that possibly have this
instruction but it was probably already broken since we use the 'ISH'
option which doesn't seem to be valid on this ISA.

This will also allow sharing headers in a multi-arch environment and
reduce the build system complexity.

Signed-off-by: Michael Jeanson <mjeanson at efficios.com>
Cc: Jason Wessel <jason.wessel at windriver.com>
Change-Id: I8e56ada55148d8e0f198c3d2e741ea414de5fef2
---
 configure.ac             | 19 -------------------
 include/urcu/arch.h      |  7 +++++++
 include/urcu/arch/arm.h  | 13 +++++++++++--
 include/urcu/config.h.in |  3 ---
 4 files changed, 18 insertions(+), 24 deletions(-)

diff --git a/configure.ac b/configure.ac
index d1d43e6..daa967a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -21,7 +21,6 @@ m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 AC_REQUIRE_AUX_FILE([tap-driver.sh])
 
 AH_TEMPLATE([CONFIG_RCU_SMP], [Enable SMP support. With SMP support enabled, uniprocessors are also supported. With SMP support disabled, UP systems work fine, but the behavior of SMP systems is undefined.])
-AH_TEMPLATE([CONFIG_RCU_ARM_HAVE_DMB], [Use the dmb instruction if available for use on ARM.])
 AH_TEMPLATE([CONFIG_RCU_TLS], [TLS provided by the compiler.])
 AH_TEMPLATE([CONFIG_RCU_HAVE_CLOCK_GETTIME], [clock_gettime() is detected.])
 AH_TEMPLATE([CONFIG_RCU_FORCE_SYS_MEMBARRIER], [Require the operating system to support the membarrier system call for default and bulletproof flavors.])
@@ -124,24 +123,6 @@ AS_IF([test "$host_cpu" = "armv7l"],[
 	AM_CFLAGS="$AM_CFLAGS -mcpu=cortex-a9 -mtune=cortex-a9 -O1"
 ])
 
-# ARM-specific checks
-AS_CASE([$host_cpu], [arm*], [
-	AC_MSG_CHECKING([for dmb instruction])
-	AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
-				int main()
-				{
-					asm volatile("dmb":::"memory");
-					return 0;
-				}
-		]])
-	],[
-		AC_MSG_RESULT([yes])
-		AC_DEFINE([CONFIG_RCU_ARM_HAVE_DMB], [1])
-	],[
-		AC_MSG_RESULT([no])
-	])
-])
-
 # Search for clock_gettime
 AC_SEARCH_LIBS([clock_gettime], [rt], [
 	AC_DEFINE([CONFIG_RCU_HAVE_CLOCK_GETTIME], [1])
diff --git a/include/urcu/arch.h b/include/urcu/arch.h
index c4b8bc2..620743c 100644
--- a/include/urcu/arch.h
+++ b/include/urcu/arch.h
@@ -41,6 +41,7 @@
  * URCU_ARCH_ALPHA : All DEC Alpha variants
  * URCU_ARCH_IA64 : All Intel Itanium variants
  * URCU_ARCH_ARM : All ARM 32 bits variants
+ *   URCU_ARCH_ARMV7 : All ARMv7 ISA variants
  * URCU_ARCH_AARCH64 : All ARM 64 bits variants
  * URCU_ARCH_MIPS : All MIPS variants
  * URCU_ARCH_NIOS2 : All Intel / Altera NIOS II variants
@@ -105,6 +106,12 @@
 #define URCU_ARCH_IA64 1
 #include <urcu/arch/ia64.h>
 
+#elif (defined(__ARM_ARCH_7A__) || defined(__ARM_ARCH_7__))
+
+#define URCU_ARCH_ARMV7 1
+#define URCU_ARCH_ARM 1
+#include <urcu/arch/arm.h>
+
 #elif (defined(__arm__) || defined(__arm))
 
 #define URCU_ARCH_ARM 1
diff --git a/include/urcu/arch/arm.h b/include/urcu/arch/arm.h
index e904b06..54ca4fa 100644
--- a/include/urcu/arch/arm.h
+++ b/include/urcu/arch/arm.h
@@ -30,7 +30,15 @@
 extern "C" {
 #endif
 
-#ifdef CONFIG_RCU_ARM_HAVE_DMB
+/*
+ * Using DMB is faster than the builtin __sync_synchronize and this instruction is
+ * part of the baseline ARMv7 ISA.
+ */
+#ifdef URCU_ARCH_ARMV7
+
+/* For backwards compat. */
+#define CONFIG_RCU_ARM_HAVE_DMB 1
+
 /*
  * Issues full system DMB operation.
  */
@@ -44,7 +52,8 @@ extern "C" {
 #define cmm_smp_mb()	__asm__ __volatile__ ("dmb ish":::"memory")
 #define cmm_smp_rmb()	__asm__ __volatile__ ("dmb ish":::"memory")
 #define cmm_smp_wmb()	__asm__ __volatile__ ("dmb ish":::"memory")
-#endif /* CONFIG_RCU_ARM_HAVE_DMB */
+
+#endif /* URCU_ARCH_ARMV7 */
 
 #include <stdlib.h>
 #include <sys/time.h>
diff --git a/include/urcu/config.h.in b/include/urcu/config.h.in
index faf7817..99d763a 100644
--- a/include/urcu/config.h.in
+++ b/include/urcu/config.h.in
@@ -5,9 +5,6 @@
    behavior of SMP systems is undefined. */
 #undef CONFIG_RCU_SMP
 
-/* Use the dmb instruction is available for use on ARM. */
-#undef CONFIG_RCU_ARM_HAVE_DMB
-
 /* TLS provided by the compiler. */
 #undef CONFIG_RCU_TLS
 
-- 
2.29.2