On Wed, May 27, 2020 at 1:14 PM Nick Desaulniers <ndesaulni...@google.com> wrote: > > On Wed, May 27, 2020 at 12:28 PM Robin Murphy <robin.mur...@arm.com> wrote: > > > > On 2020-05-27 18:55, Nick Desaulniers wrote: > > > On Wed, May 27, 2020 at 6:45 AM Robin Murphy <robin.mur...@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
