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    SubjectRe: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm
    On Wed, May 27, 2020 at 1:14 PM Nick Desaulniers
    <ndesaulniers@google.com> wrote:
    >
    > 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)

    And maybe some other useful data points regarding warning on use of r7
    and frame pointers.
    https://github.com/ClangBuiltLinux/linux/issues/701#issuecomment-591325758
    https://bugs.llvm.org/show_bug.cgi?id=45826
    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94986

    + Peter (ARM)
    + David, Arnd (Linaro)
    --
    Thanks,
    ~Nick Desaulniers

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