eal: add cache line demotion API

rte_cldemote is similar to a prefetch hint - in reverse.
On x86, cldemote(addr) enables software to hint to hardware that line is
likely to be shared. This is quite 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@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: David Christensen <drc@linux.vnet.ibm.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>

diff --git a/app/test/test_prefetch.c b/app/test/test_prefetch.c
index 32e08f8..5489885 100644 (file)
--- a/app/test/test_prefetch.c
+++ b/app/test/test_prefetch.c
@@ -30,6 +30,8 @@ test_prefetch(void)
        rte_prefetch1_write(&a);
        rte_prefetch2_write(&a);
 
+       rte_cldemote(&a);
+
        return 0;
 }
 

diff --git a/doc/guides/rel_notes/release_20_11.rst b/doc/guides/rel_notes/release_20_11.rst
index cda5b2f..48717ee 100644 (file)
--- a/doc/guides/rel_notes/release_20_11.rst
+++ b/doc/guides/rel_notes/release_20_11.rst
@@ -68,6 +68,15 @@ New Features
   which allow the programmer to prefetch a cache line and also indicate
   the intention to write.
 
+* **Added the rte_cldemote API.**
+
+  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.
+  This API is specific to x86 and implemented as a stub for other
+  architectures.
+
 * **Updated CRC modules of the net library.**
 
   * Added runtime selection of the optimal architecture-specific CRC path.

diff --git a/lib/librte_eal/arm/include/rte_prefetch_32.h b/lib/librte_eal/arm/include/rte_prefetch_32.h
index e53420a..303caaa 100644 (file)
--- a/lib/librte_eal/arm/include/rte_prefetch_32.h
+++ b/lib/librte_eal/arm/include/rte_prefetch_32.h
@@ -33,6 +33,13 @@ 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..e28b66f 100644 (file)
--- a/lib/librte_eal/arm/include/rte_prefetch_64.h
+++ b/lib/librte_eal/arm/include/rte_prefetch_64.h
@@ -32,6 +32,13 @@ 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 df9764e..f9fab5e 100644 (file)
--- a/lib/librte_eal/include/generic/rte_prefetch.h
+++ b/lib/librte_eal/include/generic/rte_prefetch.h
@@ -116,4 +116,22 @@ rte_prefetch2_write(const void *p)
        __builtin_prefetch(p, 1, 1);
 }
 
+/**
+ * @warning
+ * @b EXPERIMENTAL: this API may change, or be removed, without prior notice
+ *
+ * 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 guaranteed. 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..6df8087 100644 (file)
--- a/lib/librte_eal/ppc/include/rte_prefetch.h
+++ b/lib/librte_eal/ppc/include/rte_prefetch.h
@@ -34,6 +34,13 @@ 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..5340498 100644 (file)
--- a/lib/librte_eal/x86/include/rte_prefetch.h
+++ b/lib/librte_eal/x86/include/rte_prefetch.h
@@ -32,6 +32,17 @@ static inline void rte_prefetch_non_temporal(const volatile void *p)
        asm volatile ("prefetchnta %[p]" : : [p] "m" (*(const volatile char *)p));
 }
 
+/*
+ * We use 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