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

[Nick Desaulniers]

On Wed, May 27, 2020 at 1:14 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> On Wed, May 27, 2020 at 12:28 PM Robin Murphy <robin.murphy@xxxxxxx> wrote:
> >
> > On 2020-05-27 18:55, Nick Desaulniers wrote:
> > > On Wed, May 27, 2020 at 6:45 AM Robin Murphy <robin.murphy@xxxxxxx> 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