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

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Message-ID: <d2bf55cf-8b72-3f88-3c0b-5dfb4e120f7f@arm.com>
Date:   Wed, 27 May 2020 21:02:33 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Naohiro Aota <naohiro.aota@....com>,
        Stephen Boyd <swboyd@...gle.com>,
        Catalin Marinas <catalin.marinas@....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>,
        Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm

On 2020-05-27 20:28, Robin Murphy wrote:
> On 2020-05-27 18:55, 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?
> 
> 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. I 

Oh, and for the avoidance of ambiguity that's "Arm" as in the 32-bit Arm 
architecture port, not a specific instruction set.

Robin.

> 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?
> 
>> What is the implicit default of your gcc-arm-linux-gnueabihf at -O2?
>> -mthumb, or -marm?
> 
> As Dave pointed out, like the probable majority of users it's Thumb:
> 
> $ arm-linux-gnueabihf-gcc -v
> Using built-in specs.
> COLLECT_GCC=arm-linux-gnueabihf-gcc
> COLLECT_LTO_WRAPPER=/usr/lib/gcc-cross/arm-linux-gnueabihf/8/lto-wrapper
> Target: arm-linux-gnueabihf
> Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 
> 8.3.0-6ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-8/README.Bugs 
> --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++ --prefix=/usr 
> --with-gcc-major-version-only --program-suffix=-8 --enable-shared 
> --enable-linker-build-id --libexecdir=/usr/lib 
> --without-included-gettext --enable-threads=posix --libdir=/usr/lib 
> --enable-nls --with-sysroot=/ --enable-clocale=gnu 
> --enable-libstdcxx-debug --enable-libstdcxx-time=yes 
> --with-default-libstdcxx-abi=new --enable-gnu-unique-object 
> --disable-libitm --disable-libquadmath --disable-libquadmath-support 
> --enable-plugin --enable-default-pie --with-system-zlib 
> --with-target-system-zlib --enable-multiarch --enable-multilib 
> --disable-sjlj-exceptions --with-arch=armv7-a --with-fpu=vfpv3-d16 
> --with-float=hard --with-mode=thumb --disable-werror --enable-multilib 
> --enable-checking=release --build=aarch64-linux-gnu 
> --host=aarch64-linux-gnu --target=arm-linux-gnueabihf 
> --program-prefix=arm-linux-gnueabihf- 
> --includedir=/usr/arm-linux-gnueabihf/include
> Thread model: posix
> gcc version 8.3.0 (Ubuntu/Linaro 8.3.0-6ubuntu1)
> 
> (yeah, I didn't actually need to hack my makefile at all)
> 
> Robin.
> 
> _______________________________________________
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> linux-arm-kernel@...ts.infradead.org
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