[PATCH] RISC-V: Fix for icache flush issue on multicore processors.

[Jim Wilson <jimw at sifive dot com>]

The RISC-V ISA defines fence.i as flushing the icache on the current hart
(hardware thread, aka core) and not on any other hart.  If the linux kernel
moves a process to a different hart in between writes to code, and the
following fence.i instruction, then the result is indeterminate.  To fix this,
we need to add a new kernel interface to flush the icache, which is currently
implemented as a VDSO.  Since the glibc support has not been contributed yet,
we need to fix this now before doing so creates a linux ABI change.

This was tested with a riscv64 linux make check.  Since I'm using a user-mode
qemu to test, and user-mode qemu doesn't have VDSO support (yet?) that means
every testcase that requires an icache flush is failing.  This is a temporary
problem that we have to live with for now.  Otherwise, there are no
regressions.

Committed.

	gcc/
	* config/riscv/linux.h (ICACHE_FLUSH_FUNC): New.
	* config/riscv/riscv.md (clear_cache): Use it.
---
 gcc/config/riscv/linux.h  | 2 ++
 gcc/config/riscv/riscv.md | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/gcc/config/riscv/linux.h b/gcc/config/riscv/linux.h
index 6c7e3c4e819..4b2f7b6e1fd 100644
--- a/gcc/config/riscv/linux.h
+++ b/gcc/config/riscv/linux.h
@@ -45,6 +45,8 @@ along with GCC; see the file COPYING3.  If not see
 #define LIB_SPEC GNU_USER_TARGET_LIB_SPEC " -latomic "
 #endif
 
+#define ICACHE_FLUSH_FUNC "__riscv_flush_icache"
+
 #define LINK_SPEC "\
 -melf" XLEN_SPEC "lriscv \
 %{shared} \
diff --git a/gcc/config/riscv/riscv.md b/gcc/config/riscv/riscv.md
index db4fed48e53..dab54ad2738 100644
--- a/gcc/config/riscv/riscv.md
+++ b/gcc/config/riscv/riscv.md
@@ -1456,7 +1456,13 @@
    (match_operand 1 "pmode_register_operand")]
   ""
 {
+#ifdef ICACHE_FLUSH_FUNC
+  emit_library_call (gen_rtx_SYMBOL_REF (Pmode, ICACHE_FLUSH_FUNC),
+		     LCT_NORMAL, VOIDmode, operands[0], Pmode,
+		     operands[1], Pmode, const0_rtx, Pmode);
+#else
   emit_insn (gen_fence_i ());
+#endif
   DONE;
 })
 
-- 
2.14.1