From: Nick Desaulniers <ndesaulniers@google.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
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>,
Kristof Beyls <Kristof.Beyls@arm.com>,
victor.campos@arm.com
Subject: Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm
Date: Wed, 27 May 2020 13:14:48 -0700 [thread overview]
Message-ID: <CAKwvOdn1W4C5HRJEch5PS-Atcmysh0UD+VZX_wi8tviGwhmM7Q@mail.gmail.com> (raw)
In-Reply-To: <34f261f7-c4b5-a628-9a4c-eb97b75fba52@arm.com>
On Wed, May 27, 2020 at 12:28 PM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2020-05-27 18:55, 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?
>
> It can be, if you choose to build with frame pointers and the common
> frame pointer ABI for Thumb code that uses r7. However it can also be
> for other things like the syscall number in the Arm syscall ABI too.
Ah, right, with -fomit-frame-pointer, this error also goes away. Not
sure if we prefer either:
- build the compat vdso as -marm always or
- disable frame pointers for the vdso (does this have unwinding implications?)
- other?
> I
> take it Clang has decided that writing syscall wrappers with minimal
> inline asm is not a thing people deserve to do without arbitrary other
> restrictions?
Was the intent not obvious? We would have gotten away with it, too, if
wasn't for you meddling kids and your stupid dog! /s
https://www.youtube.com/watch?v=hXUqwuzcGeU
Anyways, this seems to explain more the intentions:
https://reviews.llvm.org/D76848#1945810
+ Victor, Kristof (ARM)
--
Thanks,
~Nick Desaulniers
next prev parent reply other threads:[~2020-05-27 20:15 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
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 [this message]
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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