On Wed, May 27, 2020 at 10:55:24AM -0700, 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? > > What is the implicit default of your gcc-arm-linux-gnueabihf at -O2? > -mthumb, or -marm?
Hmm, but this *is* weird because if I build a 32-bit kernel then I get either an ARM or a Thumb-2 VDSO depending on CONFIG_THUMB2_KERNEL. I'm not sure if that's deliberate, but both build and appear to work. I'll drop this patch for now, while we figure it out a bit more. Cheers, Will
