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    SubjectRe: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm
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    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@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. 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@lists.infradead.org
    > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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