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RISC-V: build: Allow LTO to be selected

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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>
This commit is contained in:
Wende Tan 2023-10-17 15:21:04 -07:00 committed by Palmer Dabbelt
parent 3a6dd5f614
commit 021d23428b
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3 changed files with 11 additions and 0 deletions
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3
arch/riscv/Kconfig
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@ -47,6 +47,9 @@ config RISCV
select ARCH_SUPPORTS_CFI_CLANG
select ARCH_SUPPORTS_DEBUG_PAGEALLOC if MMU
select ARCH_SUPPORTS_HUGETLBFS if MMU
# LLD >= 14: https://github.com/llvm/llvm-project/issues/50505
select ARCH_SUPPORTS_LTO_CLANG if LLD_VERSION >= 140000
select ARCH_SUPPORTS_LTO_CLANG_THIN if LLD_VERSION >= 140000
select ARCH_SUPPORTS_PAGE_TABLE_CHECK if MMU
select ARCH_SUPPORTS_PER_VMA_LOCK if MMU
select ARCH_SUPPORTS_SHADOW_CALL_STACK if HAVE_SHADOW_CALL_STACK

5
arch/riscv/Makefile
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@ -50,6 +50,11 @@ ifndef CONFIG_AS_IS_LLVM
KBUILD_CFLAGS += -Wa,-mno-relax
KBUILD_AFLAGS += -Wa,-mno-relax
endif
# LLVM has an issue with target-features and LTO: https://github.com/llvm/llvm-project/issues/59350
# Ensure it is aware of linker relaxation with LTO, otherwise relocations may
# be incorrect: https://github.com/llvm/llvm-project/issues/65090
else ifeq ($(CONFIG_LTO_CLANG),y)
KBUILD_LDFLAGS += -mllvm -mattr=+c -mllvm -mattr=+relax
endif
ifeq ($(CONFIG_SHADOW_CALL_STACK),y)

3
arch/riscv/kernel/pi/Makefile
View File

@ -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