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

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Message-ID: <CAMj1kXFK6nf3YM8d0Tfn+yBRnHCrNj3rGON9pdV7EuCGPEVpTg@mail.gmail.com>
Date: Tue, 20 Feb 2024 09:49:58 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Arnd Bergmann <arnd@...db.de>, 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 Tue, 20 Feb 2024 at 08:55, Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Mon, 19 Feb 2024 at 19:22, Mark Rutland <mark.rutland@....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@...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+.
> >
>
> 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.

Another data point: __GCC_ASM_FLAG_OUTPUTS__ needs GCC 6 or later on x86.