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

Nick Desaulniers
2020-05-27T21:48:02Z

On Wed, May 27, 2020 at 1:31 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> 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)

Also, when I looked into this briefly, I didn't happen to see anything
in AAPCS that mentions r7 is used as the frame pointer for THUMB.
Does AAPCS not cover THUMB? It also states the TPCS is obsolete.
https://developer.arm.com/docs/ihi0042/latest
https://static.docs.arm.com/ihi0042/i/aapcs32.pdf

--
Thanks,
~Nick Desaulniers