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

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Date:   Wed, 27 May 2020 20:28:12 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        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 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 
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.