Re: [PATCH v2] KVM: arm64: nvhe: Fix build with profile optimization

[Denis Nikitin]

Thank you Marc for figuring out the filtering-out solution!
It fixed the build on ChromeOS.

I will update the patch and also filter out `-fprofile-use` which will avoid
a similar problem with the instrumented PGO in the future.

Thanks,
Denis

On Thu, Oct 13, 2022 at 4:09 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Tue, 11 Oct 2022 03:15:36 +0100,
> Denis Nikitin <denik@xxxxxxxxxxxx> wrote:
> >
> > On Sat, Oct 8, 2022 at 7:22 PM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > >
> > > On Thu, 06 Oct 2022 17:28:17 +0100,
> > > Denis Nikitin <denik@xxxxxxxxxxxx> wrote:
> > > >
> > > > Hi Mark,
> > >
> > > s/k/c/
> > >
> > > >
> > > > This problem currently blocks the PGO roll on the ChromeOS kernel and
> > > > we need some kind of a solution.
> > >
> > > I'm sorry, but I don't feel constrained by your internal deadlines. I
> > > have my own...
> > >
> > > > Could you please take a look?
> > >
> > > I have asked for a reproducer. All I got for an answer is "this is
> > > hard". Providing a profiling file would help, for example.
> >
> > Could you please try the following profile on the 5.15 branch?
> >
> > $ cat <<EOF > prof.txt
> > kvm_pgtable_walk:100:10
> >  2: 5
> >  3: 5
> >  5: 5
> >  6: 5
> >  10: 5
> >  10: _kvm_pgtable_walk:50
> >   5: 5
> >   7: 5
> >   10: 5
> >   13.2: 5
> >   14: 5
> >   16: 5 __kvm_pgtable_walk:5
> >   13: kvm_pgd_page_idx:30
> >    2: __kvm_pgd_page_idx:30
> >     2: 5
> >     3: 5
> >     5: 5
> >     2: kvm_granule_shift:5
> >      3: 5
> > EOF
> >
> > $ make LLVM=1 ARCH=arm64 KCFLAGS=-fprofile-sample-use=prof.txt -j8 vmlinux
>
> Thanks, this was helpful, as I was able to reproduce the build failure.
>
> FWIW, it seems pretty easy to work around by filtering out the
> offending option, making it consistent with the mechanism we already
> use for tracing and the like.
>
> I came up with the hack below, which does the trick and is IMHO better
> than dropping the section (extra work) or adding the negation of this
> option (which depends on the compiler option evaluation order).
>
>         M.
>
> diff --git a/arch/arm64/kvm/hyp/nvhe/Makefile b/arch/arm64/kvm/hyp/nvhe/Makefile
> index 48f6ae7cc6e6..7df1b6afca7f 100644
> --- a/arch/arm64/kvm/hyp/nvhe/Makefile
> +++ b/arch/arm64/kvm/hyp/nvhe/Makefile
> @@ -91,7 +91,7 @@ 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
>
>
> --
> Without deviation from the norm, progress is not possible.