linux-kernel - Re: [PATCH v2] arm64: jump_label: use constraints "Si" instead of "i"

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Date: Mon, 19 Feb 2024 18:22:21 +0000
From: Mark Rutland <mark.rutland@....com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Ard Biesheuvel <ardb@...nel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Fangrui Song <maskray@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, linux-arm-kernel@...ts.infradead.org,
	Jisheng Zhang <jszhang@...nel.org>,
	Dave Martin <Dave.Martin@....com>,
	Peter Smith <peter.smith@....com>, llvm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64: jump_label: use constraints "Si" instead of "i"

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@...ux-m68k.org> wrote:
> >> On Mon, Feb 19, 2024 at 11:57 AM Ard Biesheuvel <ardb@...nel.org> wrote:
> >> > On Mon, 19 Feb 2024 at 11:56, Ard Biesheuvel <ardb@...nel.org> wrote:
> 
> >> > 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 also
remove some constraints on other features (e.g. the mcount-based form's graph
tracer isn't compatible with pointer authentication), it would simplify a few
things going forwards (e.g. the implementation of RELIABLE_STACKTRACE, since we
could rely on having ftrace_regs and a single trampoline), and the remaining
support would be better tested.

I've wanted to remove the old ftrace implementation for a while, but on its own
it was never important/urgent enough to justify bumping to GCC 8+.

Mark.