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From: Ard Biesheuvel <ardb@kernel.org>
Date: Tue, 20 Feb 2024 08:55:21 +0100
Message-ID: <CAMj1kXGWiTusoFWPCwM9OVwxp5=BHPfUdG-CurO13mWOdQr6Hg@mail.gmail.com>
Subject: Re: [PATCH v2] arm64: jump_label: use constraints "Si" instead of "i"
To: Mark Rutland <mark.rutland@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>, Geert Uytterhoeven <geert@linux-m68k.org>, 
	Fangrui Song <maskray@google.com>, Catalin Marinas <catalin.marinas@arm.com>, 
	Will Deacon <will@kernel.org>, linux-arm-kernel@lists.infradead.org, 
	Jisheng Zhang <jszhang@kernel.org>, Dave Martin <Dave.Martin@arm.com>, 
	Peter Smith <peter.smith@arm.com>, llvm@lists.linux.dev, linux-kernel@vger.kernel.org
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On Mon, 19 Feb 2024 at 19:22, Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Mon, Feb 19, 2024 at 06:06:19PM +0100, Arnd Bergmann wrote:
> > On Mon, Feb 19, 2024, at 16:41, Ard Biesheuvel wrote:
> > > On Mon, 19 Feb 2024 at 15:43, Geert Uytterhoeven <geert@linux-m68k.or=
g> wrote:
> > >> On Mon, Feb 19, 2024 at 11:57=E2=80=AFAM Ard Biesheuvel <ardb@kernel=
org> wrote:
> > >> > On Mon, 19 Feb 2024 at 11:56, Ard Biesheuvel <ardb@kernel.org> wro=
te:
> >
> > >> > https://godbolt.org/z/GTnf3vPaT
> > >>
> > >> I could reproduce the issue on v6.8-rc5 using arm64 defconfig
> > >> and x86_64-gcc-5.5.0-nolibc-aarch64-linux.tar.xz from
> > >> https://cdn.kernel.org/pub/tools/crosstool/files/bin/x86_64/5.5.0/:
> > >>
> > >
> > > OK, I managed to do so as well.
> > >
> > > And GCC 6.4 from the same source works correctly.
> > >
> > > Not sure whether there are any plans to bump the minimal GCC version
> > > any time soon (cc'ing Arnd), but we should probably drop this change
> > > until that happens.
> >
> > From what I can tell, we may as well formally raise the minimum
> > gcc version to 8.1+ already, as that is a version that is
> > actually used in distros, and we have been on 5.1+ for a few
> > years already.
> >
> > Not sure if there are any other benefits to gcc-8 besides
> > allowing minor cleanups.
>
> Arguably a minor cleanup, but on arm64 that'd allow us to get rid of the =
old
> mcount-based ftrace implementation and rely on -fpatchable-function-entry=
.
> On its own that'd save ~130 lines of asm and ~70 lines of C, but it'd als=
o
> remove some constraints on other features (e.g. the mcount-based form's g=
raph
> tracer isn't compatible with pointer authentication), it would simplify a=
 few
> things going forwards (e.g. the implementation of RELIABLE_STACKTRACE, si=
nce we
> could rely on having ftrace_regs and a single trampoline), and the remain=
ing
> support would be better tested.
>
> I've wanted to remove the old ftrace implementation for a while, but on i=
ts own
> it was never important/urgent enough to justify bumping to GCC 8+.
>

I don't think this is minor, tbh. Supporting two versions of the
highly complex tracing infrastructure for a toolchain that is only
used in CI seems like a waste of time and effort.

I checked x86, and it needs at least GCC 7 for retpoline support, so I
reckon at least GCC 5/6 support might be dropped there as well.