[PATCH v6] eal/x86: add 128-bit atomic compare exchange

[Gage Eads <gage.eads@intel.com>]

This operation can be used for non-blocking algorithms, such as a
non-blocking stack or ring.

It is available only for x86_64.

Signed-off-by: Gage Eads <gage.eads@intel.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
---
This patch addresses x86-64 only; other architectures can/will be supported
in the future. The __atomic intrinsic was considered for the implementation,
however libatomic was found[1] to use locks to implement the 128-bit CAS on at
least one architecture and so is eschewed here. The interface is modeled after
the __atomic_compare_exchange_16 (which itself is based on the C++11 memory
model) to best support weak consistency architectures.

This patch was originally part of a series that introduces a non-blocking stack
mempool handler[2], and is required by a non-blocking ring patchset. This
patch was spun off so that the the NB ring depends only on this patch
and not on the entire non-blocking stack patchset.

[1] http://mails.dpdk.org/archives/dev/2019-January/124002.html
[2] http://mails.dpdk.org/archives/dev/2019-January/123653.html

v6:
- Use @code and @endcode to clean up pseudo-code generation
- Add note that the function is currently limited to x86-64

v5:
 - Move declaration in the generic file for doxygen only (Thomas)

v4:
 - Move function declaration from generic/rte_atomic.h to x86-64 header file

v3:
 - Rename function to ISA-neutral rte_atomic128_cmp_exchange()
 - Fix two pseudocode bugs in function documentation

v2:
 - Rename function to rte_atomic128_cmpxchg()
 - Replace "=A" output constraint with "=a" and "=d" to prevent GCC from using
   the al register for the sete destination
 - Extend 'weak' definition to allow non-atomic 'exp' updates.
 - Add const keyword to 'src' and remove volatile keyword from 'dst'
 - Put __int128 in a union in rte_int128_t and move the structure definition
   inside the RTE_ARCH_x86_64 ifdef
 - Drop enum rte_atomic_memmodel_t in favor of compiler-defined __ATOMIC_*
 - Drop unnecessary comment relating to X86_64
 - Tweak the pseudocode to reflect the 'exp' update on failure.

 .../common/include/arch/x86/rte_atomic_64.h        | 47 +++++++++++++++++++
 lib/librte_eal/common/include/generic/rte_atomic.h | 52 ++++++++++++++++++++++
 2 files changed, 99 insertions(+)

diff --git a/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h b/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h
index fd2ec9c53..4b8315926 100644
--- a/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h
+++ b/lib/librte_eal/common/include/arch/x86/rte_atomic_64.h
@@ -34,6 +34,7 @@
 /*
  * Inspired from FreeBSD src/sys/amd64/include/atomic.h
  * Copyright (c) 1998 Doug Rabson
+ * Copyright (c) 2019 Intel Corporation
  * All rights reserved.
  */
 
@@ -46,6 +47,7 @@
 
 #include <stdint.h>
 #include <rte_common.h>
+#include <rte_compat.h>
 #include <rte_atomic.h>
 
 /*------------------------- 64 bit atomic operations -------------------------*/
@@ -208,4 +210,49 @@ static inline void rte_atomic64_clear(rte_atomic64_t *v)
 }
 #endif
 
+/*------------------------ 128 bit atomic operations -------------------------*/
+
+/**
+ * 128-bit integer structure.
+ */
+RTE_STD_C11
+typedef struct {
+	RTE_STD_C11
+	union {
+		uint64_t val[2];
+		__int128 int128;
+	};
+} __rte_aligned(16) rte_int128_t;
+
+static inline int __rte_experimental
+rte_atomic128_cmp_exchange(rte_int128_t *dst,
+			   rte_int128_t *exp,
+			   const rte_int128_t *src,
+			   unsigned int weak,
+			   int success,
+			   int failure)
+{
+	RTE_SET_USED(weak);
+	RTE_SET_USED(success);
+	RTE_SET_USED(failure);
+	uint8_t res;
+
+	asm volatile (
+		      MPLOCKED
+		      "cmpxchg16b %[dst];"
+		      " sete %[res]"
+		      : [dst] "=m" (dst->val[0]),
+			"=a" (exp->val[0]),
+			"=d" (exp->val[1]),
+			[res] "=r" (res)
+		      : "b" (src->val[0]),
+			"c" (src->val[1]),
+			"a" (exp->val[0]),
+			"d" (exp->val[1]),
+			"m" (dst->val[0])
+		      : "memory");
+
+	return res;
+}
+
 #endif /* _RTE_ATOMIC_X86_64_H_ */
diff --git a/lib/librte_eal/common/include/generic/rte_atomic.h b/lib/librte_eal/common/include/generic/rte_atomic.h
index e91742702..99585436c 100644
--- a/lib/librte_eal/common/include/generic/rte_atomic.h
+++ b/lib/librte_eal/common/include/generic/rte_atomic.h
@@ -1079,4 +1079,56 @@ static inline void rte_atomic64_clear(rte_atomic64_t *v)
 }
 #endif
 
+/*------------------------ 128 bit atomic operations -------------------------*/
+
+#ifdef __DOXYGEN__
+
+/**
+ * An atomic compare and set function used by the mutex functions.
+ * (Atomically) Equivalent to:
+ * @code
+ *   if (*dst == *exp)
+ *     *dst = *src
+ *   else
+ *     *exp = *dst
+ * @endcode
+ *
+ * @note This function is currently only available for the x86-64 platform.
+ *
+ * @note The success and failure arguments must be one of the __ATOMIC_* values
+ * defined in the C++11 standard. For details on their behavior, refer to the
+ * standard.
+ *
+ * @param dst
+ *   The destination into which the value will be written.
+ * @param exp
+ *   Pointer to the expected value. If the operation fails, this memory is
+ *   updated with the actual value.
+ * @param src
+ *   Pointer to the new value.
+ * @param weak
+ *   A value of true allows the comparison to spuriously fail and allows the
+ *   'exp' update to occur non-atomically (i.e. a torn read may occur).
+ *   Implementations may ignore this argument and only implement the strong
+ *   variant.
+ * @param success
+ *   If successful, the operation's memory behavior conforms to this (or a
+ *   stronger) model.
+ * @param failure
+ *   If unsuccessful, the operation's memory behavior conforms to this (or a
+ *   stronger) model. This argument cannot be __ATOMIC_RELEASE,
+ *   __ATOMIC_ACQ_REL, or a stronger model than success.
+ * @return
+ *   Non-zero on success; 0 on failure.
+ */
+static inline int __rte_experimental
+rte_atomic128_cmp_exchange(rte_int128_t *dst,
+			   rte_int128_t *exp,
+			   const rte_int128_t *src,
+			   unsigned int weak,
+			   int success,
+			   int failure);
+
+#endif /* __DOXYGEN__ */
+
 #endif /* _RTE_ATOMIC_H_ */
-- 
2.13.6