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>

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index bffbd86..60c8fd1 100644 (file)
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -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

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 0b7d109..252d639 100644 (file)
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -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)

diff --git a/arch/riscv/kernel/pi/Makefile b/arch/riscv/kernel/pi/Makefile
index 07915dc..b75f150 100644 (file)
--- 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