As per chapter 15 "Errata: Issue of Out-of-order in loongson"[1] to work
around the Loongson 2F erratum we need to do:
"When switching from user mode to kernel mode, you should flush the
branch target history such as BTB and RAS."
[1] Chinese version: http://www.loongson.cn/uploadfile/file/
200808211
[2] English version of chapter 15:
http://groups.google.com.hk/group/loongson-dev/msg/
e0d2e220958f10a6?dmode=source
Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Patchwork: http://patchwork.linux-mips.org/patch/1066/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
.endm
#else
.macro get_saved_sp /* Uniprocessor variation */
+#ifdef CONFIG_CPU_LOONGSON2F
+ /*
+ * Clear BTB (branch target buffer), forbid RAS (return address
+ * stack) to workaround the Out-of-order Issue in Loongson2F
+ * via its diagnostic register.
+ */
+ move k0, ra
+ jal 1f
+ nop
+1: jal 1f
+ nop
+1: jal 1f
+ nop
+1: jal 1f
+ nop
+1: move ra, k0
+ li k0, 3
+ mtc0 k0, $22
+#endif /* CONFIG_CPU_LOONGSON2F */
#if defined(CONFIG_32BIT) || defined(KBUILD_64BIT_SYM32)
lui k1, %hi(kernelsp)
#else