Please can you tell me exactly which LLVM version (including git revision, if you're on master) you are using?

HEAD, as of today (f2af4f8) does not include any code to translate -m{no-}save-restore into target features, only a warning is emitted (https://github.com/llvm/llvm-project/blob/f2af4f8a45dbd0d1de950a64f1b4fbf618c70657/clang/lib/Driver/ToolChains/Arch/RISCV.cpp#L437-441)

Have you applied some patches of your own?

No, both the LLVM toolchain and Linux kernel are completely vanilla.

$ clang --version | head -n1
ClangBuiltLinux clang version 11.0.0 (git://github.com/llvm/llvm-project 8b81ebf)

$ git describe --contains
v5.6-rc1^0

$ make -j$(nproc) -s ARCH=riscv CC=clang CROSS_COMPILE=riscv64-linux-gnu- O=out.riscv distclean defconfig init/main.o
...
'-save-restore' is not a recognized feature for this target (ignoring feature)
'-save-restore' is not a recognized feature for this target (ignoring feature)
'-save-restore' is not a recognized feature for this target (ignoring feature)
'-save-restore' is not a recognized feature for this target (ignoring feature)
'-save-restore' is not a recognized feature for this target (ignoring feature)
'-save-restore' is not a recognized feature for this target (ignoring feature)
'-save-restore' is not a recognized feature for this target (ignoring feature)
...

The full command from that translation unit, if it is relevant:

clang -Wp,-MD,init/.main.o.d -nostdinc -isystem /home/nathan/cbl/git/tc-build/install/lib/clang/11.0.0/include -I../arch/riscv/include -I./arch/riscv/include/generated -I../include -I./include -I../arch/riscv/include/uapi -I./arch/riscv/include/generated/uapi -I../include/uapi -I./include/generated/uapi -include ../include/linux/kconfig.h -include ../include/linux/compiler_types.h -D__KERNEL__ -Qunused-arguments -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE -Werror=implicit-function-declaration -Werror=implicit-int -Wno-format-security -std=gnu89 --target=riscv64-linux-gnu --prefix=/home/nathan/cbl/git/tc-build/install/bin/ --gcc-toolchain=/home/nathan/cbl/git/tc-build/install -no-integrated-as -Werror=unknown-warning-option -mabi=lp64 -march=rv64imac -mno-save-restore -DCONFIG_PAGE_OFFSET=0xffffffe000000000 -mcmodel=medany -fno-delete-null-pointer-checks -Wno-address-of-packed-member -O2 -Wframe-larger-than=2048 -fno-stack-protector -Wno-format-invalid-specifier -Wno-gnu -Wno-tautological-compare -mno-global-merge -Wno-unused-const-variable -fno-omit-frame-pointer -fno-optimize-sibling-calls -Wdeclaration-after-statement -Wvla -Wno-pointer-sign -fno-strict-overflow -fno-merge-all-constants -fno-stack-check -Werror=date-time -Werror=incompatible-pointer-types -fmacro-prefix-map=../= -Wno-initializer-overrides -Wno-format -Wno-sign-compare -Wno-format-zero-length -fno-function-sections -fno-data-sections -I ../init -I ./init -DKBUILD_MODFILE='"init/main"' -DKBUILD_BASENAME='"main"' -DKBUILD_MODNAME='"main"' -c -o init/main.o ../init/main.c

I cannot find "-save-restore" in any of the clang or llvm source code as of commit 8b81ebfe7eba089ed2016d523cc5ee9d05e957a7, or any of the clangbuiltlinux source code either (it would be in -mattr or the like).

I think maybe this warning is coming from some cached target-features somewhere, in a LLVM IR file or the like.

The "save-restore" feature has only been parts of patches until today, I believe, so I don't think any mainline clang version has been generating code with the save-restore feature yet.

That said, save-restore has now landed, so maybe now these warnings will go away on HEAD? Do let me know how you get on on today's HEAD.

Yes, these warnings are gone with my version of clang at bc316c2, I'll close this, thank you!