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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. > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@...ts.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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