From 021d23428bdbae032294e8f4a29cb53cb50ae71c Mon Sep 17 00:00:00 2001
From: Wende Tan <twd2.me@gmail.com>
Date: Tue, 17 Oct 2023 15:21:04 -0700
Subject: RISC-V: build: Allow LTO to be selected
Allow LTO to be selected for RISC-V, only when LLD >= 14, since there is
an issue [1] in prior LLD versions that prevents LLD to generate proper
machine code for RISC-V when writing `nop`s.
To avoid boot failures in QEMU [2], '-mattr=+c' and '-mattr=+relax'
need to be passed via '-mllvm' to ld.lld, as there appears to be an
issue with LLVM's target-features and LTO [3], which can result in
incorrect relocations to branch targets [4]. Once this is fixed in LLVM,
it can be made conditional on affected ld.lld versions.
Disable LTO for arch/riscv/kernel/pi, as llvm-objcopy expects an ELF
object file when manipulating the files in that subfolder, rather than
LLVM bitcode.
[1] https://github.com/llvm/llvm-project/issues/50505, resolved by LLVM
commit e63455d5e0e5 ("[MC] Use local MCSubtargetInfo in writeNops")
[2] https://github.com/ClangBuiltLinux/linux/issues/1942
[3] https://github.com/llvm/llvm-project/issues/59350
[4] https://github.com/llvm/llvm-project/issues/65090
Tested-by: Wende Tan <twd2.me@gmail.com>
Signed-off-by: Wende Tan <twd2.me@gmail.com>
Co-developed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231017-riscv-lto-v4-1-e7810b24e805@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
---
arch/riscv/kernel/pi/Makefile | 3 +++
1 file changed, 3 insertions(+)
(limited to 'arch/riscv/kernel/pi')
diff --git a/arch/riscv/kernel/pi/Makefile b/arch/riscv/kernel/pi/Makefile
index 07915dc9279e..b75f150b923d 100644
--- a/arch/riscv/kernel/pi/Makefile
+++ b/arch/riscv/kernel/pi/Makefile
@@ -9,6 +9,9 @@ KBUILD_CFLAGS := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) -fpie \
-fno-asynchronous-unwind-tables -fno-unwind-tables \
$(call cc-option,-fno-addrsig)
+# Disable LTO
+KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO), $(KBUILD_CFLAGS))
+
KBUILD_CFLAGS += -mcmodel=medany
CFLAGS_cmdline_early.o += -D__NO_FORTIFY
--
cgit