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From: Will Deacon <will@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Robin Murphy <robin.murphy@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Naohiro Aota <naohiro.aota@wdc.com>,
	Stephen Boyd <swboyd@google.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Manoj Gupta <manojgupta@google.com>,
	Luis Lozano <llozano@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm
Date: Wed, 27 May 2020 19:08:40 +0100	[thread overview]
Message-ID: <20200527180839.GA16411@willie-the-truck> (raw)
In-Reply-To: <CAKwvOd=Oy_OfRbL6-q-3CAHxWBNBKE+HkfNfgCiP726u+4dU1Q@mail.gmail.com>

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@arm.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

  reply	other threads:[~2020-05-27 18:08 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-26 17:31 [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm Nick Desaulniers
2020-05-26 18:55 ` Stephen Boyd
2020-05-26 20:45 ` Will Deacon
2020-05-27 13:53   ` Dave Martin
2020-05-27 17:58     ` Nick Desaulniers
2020-05-27 13:45 ` Robin Murphy
2020-05-27 17:55   ` Nick Desaulniers
2020-05-27 18:08     ` Will Deacon [this message]
2020-05-27 18:17       ` Nick Desaulniers
2020-05-28  7:20         ` Will Deacon
2020-06-08 20:57           ` [PATCH v2] arm64: vdso32: add CONFIG_THUMB2_COMPAT_VDSO Nick Desaulniers
2020-06-09 20:35             ` Catalin Marinas
2020-06-09 23:55               ` Nick Desaulniers
2020-06-10  8:47                 ` Will Deacon
2020-06-10 10:29                   ` Catalin Marinas
2020-06-10 10:32                     ` Will Deacon
2020-06-10 11:21             ` Will Deacon
2020-05-27 19:28     ` [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm Robin Murphy
2020-05-27 20:02       ` Robin Murphy
2020-05-27 20:14       ` Nick Desaulniers
2020-05-27 20:31         ` Nick Desaulniers
2020-05-27 21:47           ` Nick Desaulniers
2020-05-28  8:05           ` Peter Smith
2020-05-28  9:41             ` Catalin Marinas
2020-05-28 19:06               ` Nick Desaulniers

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