[dpdk-dev] [PATCH v7] eal: add cache-line demote support

[Omkar Maslekar]

rte_cldemote is similar to a prefetch hint - in reverse. cldemote(addr)
enables software to hint to hardware that line is likely to be shared.
Useful in core-to-core communications where cache-line is likely to be
shared. ARM and PPC implementation is provided with NOP and can be added
if any equivalent instructions could be used for implementation on those
architectures.

Signed-off-by: Omkar Maslekar <omkar.maslekar at intel.com>
Acked-by: Bruce Richardson <bruce.richardson at intel.com>
Acked-by: David Christensen <drc at linux.vnet.ibm.com>
Acked-by: Jerin Jacob <jerinj at marvell.com>

---
v7: fixed experimental tag

v6: marked rte_cldemote as experimental
    added rte_cldemote call in existing app/test_prefetch.c

v5: documentation updated
    fixed formatting issue in release notes
    added Acked-by: Bruce Richardson <bruce.richardson at intel.com>
*
v4: updated bold text for title and fixed margin in release notes
*
v3: fixed warning regarding whitespace
*
v2: documentation updated
---
---
 app/test/test_prefetch.c                      |  4 ++++
 doc/guides/rel_notes/release_20_11.rst        |  7 +++++++
 lib/librte_eal/arm/include/rte_prefetch_32.h  |  7 +++++++
 lib/librte_eal/arm/include/rte_prefetch_64.h  |  7 +++++++
 lib/librte_eal/include/generic/rte_prefetch.h | 15 +++++++++++++++
 lib/librte_eal/ppc/include/rte_prefetch.h     |  7 +++++++
 lib/librte_eal/x86/include/rte_prefetch.h     | 11 +++++++++++
 7 files changed, 58 insertions(+)

diff --git a/app/test/test_prefetch.c b/app/test/test_prefetch.c
index 41f219a..5c58d0c 100644
--- a/app/test/test_prefetch.c
+++ b/app/test/test_prefetch.c
@@ -26,7 +26,11 @@
 	rte_prefetch1(&a);
 	rte_prefetch2(&a);
 
+/* test for marking a line as shared to test cldemote functionality */
+	rte_cldemote(&a);
+
 	return 0;
 }
 
+
 REGISTER_TEST_COMMAND(prefetch_autotest, test_prefetch);
diff --git a/doc/guides/rel_notes/release_20_11.rst b/doc/guides/rel_notes/release_20_11.rst
index b7881f2..8a1ed01 100644
--- a/doc/guides/rel_notes/release_20_11.rst
+++ b/doc/guides/rel_notes/release_20_11.rst
@@ -171,6 +171,13 @@ New Features
   * Extern objects and functions can be plugged into the pipeline.
   * Transaction-oriented table updates.
 
+* **Added new function rte_cldemote in rte_prefetch.h.**
+
+  Added a hardware hint CLDEMOTE, which is similar to prefetch in reverse.
+  CLDEMOTE moves the cache line to the more remote cache, where it expects
+  sharing to be efficient. Moving the cache line to a level more distant from
+  the processor helps to accelerate core-to-core communication.
+
 
 Removed Items
 -------------
diff --git a/lib/librte_eal/arm/include/rte_prefetch_32.h b/lib/librte_eal/arm/include/rte_prefetch_32.h
index e53420a..28b3d48 100644
--- a/lib/librte_eal/arm/include/rte_prefetch_32.h
+++ b/lib/librte_eal/arm/include/rte_prefetch_32.h
@@ -10,6 +10,7 @@
 #endif
 
 #include <rte_common.h>
+#include <rte_compat.h>
 #include "generic/rte_prefetch.h"
 
 static inline void rte_prefetch0(const volatile void *p)
@@ -33,6 +34,12 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
 	rte_prefetch0(p);
 }
 
+__rte_experimental
+static inline void rte_cldemote(const volatile void *p)
+{
+	RTE_SET_USED(p);
+}
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/lib/librte_eal/arm/include/rte_prefetch_64.h b/lib/librte_eal/arm/include/rte_prefetch_64.h
index fc2b391..1c722eb 100644
--- a/lib/librte_eal/arm/include/rte_prefetch_64.h
+++ b/lib/librte_eal/arm/include/rte_prefetch_64.h
@@ -10,6 +10,7 @@
 #endif
 
 #include <rte_common.h>
+#include <rte_compat.h>
 #include "generic/rte_prefetch.h"
 
 static inline void rte_prefetch0(const volatile void *p)
@@ -32,6 +33,12 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
 	asm volatile ("PRFM PLDL1STRM, [%0]" : : "r" (p));
 }
 
+__rte_experimental
+static inline void rte_cldemote(const volatile void *p)
+{
+	RTE_SET_USED(p);
+}
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/lib/librte_eal/include/generic/rte_prefetch.h b/lib/librte_eal/include/generic/rte_prefetch.h
index 6e47bdf..ad9844c 100644
--- a/lib/librte_eal/include/generic/rte_prefetch.h
+++ b/lib/librte_eal/include/generic/rte_prefetch.h
@@ -51,4 +51,19 @@
  */
 static inline void rte_prefetch_non_temporal(const volatile void *p);
 
+/**
+ * Demote a cache line to a more distant level of cache from the processor.
+ *
+ * CLDEMOTE hints to hardware to move (demote) a cache line from the closest to
+ * the processor to a level more distant from the processor. It is a hint and
+ * not guarantee. rte_cldemote is intended to move the cache line to the more
+ * remote cache, where it expects sharing to be efficient and to indicate that a
+ * line may be accessed by a different core in the future.
+ *
+ * @param p
+ *   Address to demote
+ */
+__rte_experimental
+static inline void rte_cldemote(const volatile void *p);
+
 #endif /* _RTE_PREFETCH_H_ */
diff --git a/lib/librte_eal/ppc/include/rte_prefetch.h b/lib/librte_eal/ppc/include/rte_prefetch.h
index 9ba07c8..b55cac4 100644
--- a/lib/librte_eal/ppc/include/rte_prefetch.h
+++ b/lib/librte_eal/ppc/include/rte_prefetch.h
@@ -11,6 +11,7 @@
 #endif
 
 #include <rte_common.h>
+#include <rte_compat.h>
 #include "generic/rte_prefetch.h"
 
 static inline void rte_prefetch0(const volatile void *p)
@@ -34,6 +35,12 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
 	rte_prefetch0(p);
 }
 
+__rte_experimental
+static inline void rte_cldemote(const volatile void *p)
+{
+	RTE_SET_USED(p);
+}
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/lib/librte_eal/x86/include/rte_prefetch.h b/lib/librte_eal/x86/include/rte_prefetch.h
index 384c6b3..92ba05a 100644
--- a/lib/librte_eal/x86/include/rte_prefetch.h
+++ b/lib/librte_eal/x86/include/rte_prefetch.h
@@ -10,6 +10,7 @@
 #endif
 
 #include <rte_common.h>
+#include <rte_compat.h>
 #include "generic/rte_prefetch.h"
 
 static inline void rte_prefetch0(const volatile void *p)
@@ -32,6 +33,16 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
 	asm volatile ("prefetchnta %[p]" : : [p] "m" (*(const volatile char *)p));
 }
 
+/*
+ * we're using raw byte codes for now as only the newest compiler
+ * versions support this instruction natively.
+ */
+__rte_experimental
+static inline void rte_cldemote(const volatile void *p)
+{
+	asm volatile(".byte 0x0f, 0x1c, 0x06" :: "S" (p));
+}
+
 #ifdef __cplusplus
 }
 #endif
-- 
1.8.3.1