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SubjectRe: [PATCH v2] KVM: arm64: nvhe: Fix build with profile optimization
On Thu, 22 Sep 2022 06:31:45 +0100,
Denis Nikitin <denik@chromium.org> wrote:
>
> Kernel build with clang and KCFLAGS=-fprofile-sample-use fails with:
>
> error: arch/arm64/kvm/hyp/nvhe/kvm_nvhe.tmp.o: Unexpected SHT_REL
> section ".rel.llvm.call-graph-profile"
>
> Starting from 13.0.0 llvm can generate SHT_REL section, see
> https://reviews.llvm.org/rGca3bdb57fa1ac98b711a735de048c12b5fdd8086.
> gen-hyprel does not support SHT_REL relocation section.
>
> Remove ".llvm.call-graph-profile" SHT_REL relocation from kvm_nvhe
> to fix the build.
>
> Signed-off-by: Denis Nikitin <denik@chromium.org>
> ---
> V1 -> V2: Remove the relocation instead of disabling the profile-use flags.
> ---
> arch/arm64/kvm/hyp/nvhe/Makefile | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/hyp/nvhe/Makefile b/arch/arm64/kvm/hyp/nvhe/Makefile
> index b5c5119c7396..49ec950ac57b 100644
> --- a/arch/arm64/kvm/hyp/nvhe/Makefile
> +++ b/arch/arm64/kvm/hyp/nvhe/Makefile
> @@ -78,8 +78,10 @@ $(obj)/kvm_nvhe.o: $(obj)/kvm_nvhe.rel.o FORCE
>
> # The HYPREL command calls `gen-hyprel` to generate an assembly file with
> # a list of relocations targeting hyp code/data.
> +# Starting from 13.0.0 llvm emits SHT_REL section '.llvm.call-graph-profile'
> +# when profile optimization is applied. gen-hyprel does not support SHT_REL.
> quiet_cmd_hyprel = HYPREL $@
> - cmd_hyprel = $(obj)/gen-hyprel $< > $@
> + cmd_hyprel = $(OBJCOPY) -R .llvm.call-graph-profile $<; $(obj)/gen-hyprel $< > $@

I was really hoping that you'd just drop the flags from the CFLAGS
instead of removing the generated section. Something like:

diff --git a/arch/arm64/kvm/hyp/nvhe/Makefile b/arch/arm64/kvm/hyp/nvhe/Makefile
index b5c5119c7396..e5b2d43925b4 100644
--- a/arch/arm64/kvm/hyp/nvhe/Makefile
+++ b/arch/arm64/kvm/hyp/nvhe/Makefile
@@ -88,7 +88,7 @@ quiet_cmd_hypcopy = HYPCOPY $@

# Remove ftrace, Shadow Call Stack, and CFI CFLAGS.
# This is equivalent to the 'notrace', '__noscs', and '__nocfi' annotations.
-KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_CFI), $(KBUILD_CFLAGS))
+KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_CFI) -fprofile-sample-use, $(KBUILD_CFLAGS))

# KVM nVHE code is run at a different exception code with a different map, so
# compiler instrumentation that inserts callbacks or checks into the code may
However, I even failed to reproduce your problem using LLVM 14 as
packaged by Debian (if that matters, I'm using an arm64 build
machine). I build the kernel with:

$ make LLVM=1 KCFLAGS=-fprofile-sample-use -j8 vmlinux

and the offending object only contains the following sections:

arch/arm64/kvm/hyp/nvhe/kvm_nvhe.tmp.o: file format elf64-littleaarch64

Sections:
Idx Name Size VMA LMA File off Algn
0 .hyp.idmap.text 00000ae4 0000000000000000 0000000000000000 00000800 2**11
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .hyp.text 0000e988 0000000000000000 0000000000000000 00001800 2**11
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
2 .hyp.data..ro_after_init 00000820 0000000000000000 0000000000000000 00010188 2**3
CONTENTS, ALLOC, LOAD, DATA
3 .hyp.rodata 00002e70 0000000000000000 0000000000000000 000109a8 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
4 .hyp.data..percpu 00001ee0 0000000000000000 0000000000000000 00013820 2**4
CONTENTS, ALLOC, LOAD, DATA
5 .hyp.bss 00001158 0000000000000000 0000000000000000 00015700 2**3
ALLOC
6 .comment 0000001f 0000000000000000 0000000000000000 00017830 2**0
CONTENTS, READONLY
7 .llvm_addrsig 000000b8 0000000000000000 0000000000000000 0001784f 2**0
CONTENTS, READONLY, EXCLUDE
8 .altinstructions 00001284 0000000000000000 0000000000000000 00015700 2**0
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
9 __jump_table 00000960 0000000000000000 0000000000000000 00016988 2**3
CONTENTS, ALLOC, LOAD, RELOC, DATA
10 __bug_table 0000051c 0000000000000000 0000000000000000 000172e8 2**2
CONTENTS, ALLOC, LOAD, RELOC, DATA
11 __kvm_ex_table 00000028 0000000000000000 0000000000000000 00017808 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
12 .note.GNU-stack 00000000 0000000000000000 0000000000000000 00027370 2**0
CONTENTS, READONLY
So what am I missing to trigger this issue? Does it rely on something
like PGO, which is not upstream yet? A bit of handholding would be
much appreciated.

Thanks,

M.

--
Without deviation from the norm, progress is not possible.

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