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