[PATCH v3 4/4] riscv: xchg: Prefetch the destination word for sc.w

[Alexandre Ghiti]

From: Guo Ren <guoren at linux.alibaba.com>

The cost of changing a cacheline from shared to exclusive state can be
significant, especially when this is triggered by an exclusive store,
since it may result in having to retry the transaction.

This patch makes use of prefetch.w to prefetch cachelines for write
prior to lr/sc loops when using the xchg_small atomic routine.

This patch is inspired by commit 0ea366f5e1b6 ("arm64: atomics:
prefetch the destination word for write prior to stxr").

Signed-off-by: Guo Ren <guoren at linux.alibaba.com>
Signed-off-by: Guo Ren <guoren at kernel.org>
Link: https://lore.kernel.org/r/20231231082955.16516-4-guoren@kernel.org
Signed-off-by: Alexandre Ghiti <alexghiti at rivosinc.com>
---
 arch/riscv/include/asm/cmpxchg.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
index 2ec119eb147b..0b749e710216 100644
--- a/arch/riscv/include/asm/cmpxchg.h
+++ b/arch/riscv/include/asm/cmpxchg.h
@@ -13,6 +13,7 @@
 #include <asm/hwcap.h>
 #include <asm/insn-def.h>
 #include <asm/cpufeature-macros.h>
+#include <asm/processor.h>
 
 #define __arch_xchg_masked(sc_sfx, swap_sfx, prepend, sc_append,		\
 			   swap_append, r, p, n)				\
@@ -37,6 +38,7 @@
 										\
 		__asm__ __volatile__ (						\
 		       prepend							\
+		       PREFETCHW_ASM(%5)					\
 		       "0:	lr.w %0, %2\n"					\
 		       "	and  %1, %0, %z4\n"				\
 		       "	or   %1, %1, %z3\n"				\
@@ -44,7 +46,7 @@
 		       "	bnez %1, 0b\n"					\
 		       sc_append						\
 		       : "=&r" (__retx), "=&r" (__rc), "+A" (*(__ptr32b))	\
-		       : "rJ" (__newx), "rJ" (~__mask)				\
+		       : "rJ" (__newx), "rJ" (~__mask), "rJ" (__ptr32b)		\
 		       : "memory");						\
 										\
 		r = (__typeof__(*(p)))((__retx & __mask) >> __s);		\
-- 
2.39.2