MIPS: Loongson-2EF: disable fix-loongson3-llsc in compiler

Firstly, Loongson-2EF support ll/sc instructions, but
doesn't need fix-loongson3-llsc compile option.

Secondly, fix-loongson3-llsc will cause kernel startup
fail at futex_init, because compiler will add 'sync' before
'll', which will affect __ex_table.
futex_init will pass NULL uaddr parameter to
futex_atomic_cmpxchg_inatomic.
futex_atomic_cmpxchg_inatomic will access uaddr directly,
which will cause page fault exception, the exception should be
handled by __ex_table's nextinsn if the exception insn exsit in
__ex_table. Because __ex_table is affected by compiler,
the exception can not be handled, and
futex_atomic_cmpxchg_inatomic will crash.

Error code as below:
__ex_table.insn = 1b, which is 'sync' compiled with
fix-loongson3-llsc, but the actual exception instrction is ll.
So, do_page_fault will not find the correct inst in __ex_table, and
can not handle this exception.

        "1: "user_ll("%1", "%3")"               \n"
        "   bne %1, %z4, 3f             \n"
        "   .set    pop                 \n"
        "   move    $1, %z5                 \n"
        "   .set    "MIPS_ISA_ARCH_LEVEL"           \n"
        "2: "user_sc("$1", "%2")"               \n"
        "   beqz    $1, 1b                  \n"
        "3: " __SYNC_ELSE(full, loongson3_war, __WEAK_LLSC_MB) "\n"
        "   .insn                       \n"
        "   .set    pop                 \n"
        "   .section .fixup,\"ax\"              \n"
        "4: li  %0, %6                  \n"
        "   j   3b                  \n"
        "   .previous                   \n"
        "   .section __ex_table,\"a\"           \n"
        "   "__UA_ADDR "\t1b, 4b                \n"
        "   "__UA_ADDR "\t2b, 4b                \n"
        "   .previous

Signed-off-by: Lichao Liu <liulichao@loongson.cn>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/loongson2ef/Platform

@@ -8,6 +8,28 @@ cflags-$(CONFIG_CPU_LOONGSON2E) += \
 	$(call cc-option,-march=loongson2e,-march=r4600)
 cflags-$(CONFIG_CPU_LOONGSON2F) += \
 	$(call cc-option,-march=loongson2f,-march=r4600)
+#
+# Some versions of binutils, not currently mainline as of 2019/02/04, support
+# an -mfix-loongson3-llsc flag which emits a sync prior to each ll instruction
+# to work around a CPU bug (see __SYNC_loongson3_war in asm/sync.h for a
+# description).
+#
+# We disable this in order to prevent the assembler meddling with the
+# instruction that labels refer to, ie. if we label an ll instruction:
+#
+# 1: ll v0, 0(a0)
+#
+# ...then with the assembler fix applied the label may actually point at a sync
+# instruction inserted by the assembler, and if we were using the label in an
+# exception table the table would no longer contain the address of the ll
+# instruction.
+#
+# Avoid this by explicitly disabling that assembler behaviour. If upstream
+# binutils does not merge support for the flag then we can revisit & remove
+# this later - for now it ensures vendor toolchains don't cause problems.
+#
+cflags-$(CONFIG_CPU_LOONGSON2EF)	+= $(call as-option,-Wa$(comma)-mno-fix-loongson3-llsc,)
+
 # Enable the workarounds for Loongson2f
 ifdef CONFIG_CPU_LOONGSON2F_WORKAROUNDS
   ifeq ($(call as-option,-Wa$(comma)-mfix-loongson2f-nop,),)