Re: [PATCH] KVM: arm64: nvhe: Disable profile optimization

[Marc Zyngier]

Hi Denis,

On Tue, 20 Sep 2022 09:20:05 +0100,
Denis Nikitin <denik@xxxxxxxxxxxx> wrote:
> 
> Kernel build with -fprofile-sample-use raises the following failure:
> 
> error: arch/arm64/kvm/hyp/nvhe/kvm_nvhe.tmp.o: Unexpected SHT_REL
> section ".rel.llvm.call-graph-profile"

How is this flag provided? I don't see any occurrence of it in the
kernel so far.

> 
> SHT_REL is generated by the latest lld, see
> https://reviews.llvm.org/rGca3bdb57fa1ac98b711a735de048c12b5fdd8086.

Is this part of a released toolchain? If so, can you spell out the
first version where this occurs?

> Disable profile optimization in kvm/nvhe to fix the build with
> AutoFDO.

It'd be good to at least mention how AutoFDO and -fprofile-sample-use
relate to each other.

> 
> Signed-off-by: Denis Nikitin <denik@xxxxxxxxxxxx>
> ---
>  arch/arm64/kvm/hyp/nvhe/Makefile | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/Makefile b/arch/arm64/kvm/hyp/nvhe/Makefile
> index b5c5119c7396..6a6188374a52 100644
> --- a/arch/arm64/kvm/hyp/nvhe/Makefile
> +++ b/arch/arm64/kvm/hyp/nvhe/Makefile
> @@ -89,6 +89,9 @@ 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))
> +# Profile optimization creates SHT_REL section '.llvm.call-graph-profile' for
> +# the hot code. SHT_REL is currently not supported by the KVM tools.

'KVM tools' seems vague. Maybe call out the actual helper that
processes the relocations?

> +KBUILD_CFLAGS += $(call cc-option,-fno-profile-sample-use,-fno-profile-use)

Why adding these options instead of filtering out the offending option
as it is done just above?

Also, is this the only place the kernel fails to compile? The EFI stub
does similar things AFAIR, and could potentially fail the same way.

Thanks,

	M.

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