linux-kernel - Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm

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Message-ID: <20200527180839.GA16411@willie-the-truck>
Date:   Wed, 27 May 2020 19:08:40 +0100
From:   Will Deacon <will@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Naohiro Aota <naohiro.aota@....com>,
        Stephen Boyd <swboyd@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Manoj Gupta <manojgupta@...gle.com>,
        Luis Lozano <llozano@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm

On Wed, May 27, 2020 at 10:55:24AM -0700, Nick Desaulniers wrote:
> On Wed, May 27, 2020 at 6:45 AM Robin Murphy <robin.murphy@....com> wrote:
> >
> > On 2020-05-26 18:31, Nick Desaulniers wrote:
> > > Custom toolchains that modify the default target to -mthumb cannot
> > > compile the arm64 compat vdso32, as
> > > arch/arm64/include/asm/vdso/compat_gettimeofday.h
> > > contains assembly that's invalid in -mthumb.  Force the use of -marm,
> > > always.
> >
> > FWIW, this seems suspicious - the only assembly instructions I see there
> > are SWI(SVC), MRRC, and a MOV, all of which exist in Thumb for the
> > -march=armv7a baseline that we set.
> >
> > On a hunch, I've just bodged "VDSO_CFLAGS += -mthumb" into my tree and
> > built a Thumb VDSO quite happily with Ubuntu 19.04's
> > gcc-arm-linux-gnueabihf. What was the actual failure you saw?
> 
> From the link in the commit message: `write to reserved register 'R7'`
> https://godbolt.org/z/zwr7iZ
> IIUC r7 is reserved for the frame pointer in THUMB?
> 
> What is the implicit default of your gcc-arm-linux-gnueabihf at -O2?
> -mthumb, or -marm?

Hmm, but this *is* weird because if I build a 32-bit kernel then I get
either an ARM or a Thumb-2 VDSO depending on CONFIG_THUMB2_KERNEL. I'm
not sure if that's deliberate, but both build and appear to work.

I'll drop this patch for now, while we figure it out a bit more.

Cheers,

Will