linux-kernel - Re: [PATCH] ARM: makefile: pass -march=armv4 to assembler even on CPU32v3

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Date:   Tue, 2 Oct 2018 15:12:44 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>, arm@...nel.org
Subject: Re: [PATCH] ARM: makefile: pass -march=armv4 to assembler even on CPU32v3

Hey Ard,

On Tue, Oct 2, 2018 at 3:10 PM Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>
> It builds but it doesn't run, at least not when built into the kernel
> proper (which will be the case after random.c moves to this library)
>
> Your toolchain is implicitly passing --fix-v4bx to the assembler,
> which causes it to permit bx instructions in ARMv4 object code, but
> tag them with special R_ARM_V4BX ELF relocations. The ARM module
> loader does take these into account, so built as a module, it works.
> However, when built into the core kernel, we have to rely on the
> linker to patch this instruction into the ARMv4 equivalent, and a
> quick check reveals that that is currently not the case.
>
> Bottom line: let's not go there.

Oh, thanks a bunch for explaining that. Since I wrote the last email
I've been playing around with as, readelf, and qemu and was slightly
mystified.

>
> > But anyway,
> > I'll go with your primary suggestion and we therefore can move ahead
> > with changing the global cflag to march=armv3m. Would you like me to
> > submit the patch for this, or would yo like to handle it?
> >
>
> Yes please go ahead.

Alright, incoming in a few minutes.

Jason