On Wed, May 27, 2020 at 11:08 AM Will Deacon <w...@kernel.org> wrote: > > 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.
That's because there's 3 VDSO's when it comes to ARM: arm64's 64b vdso: arch/arm64/kernel/vdso arm64's 32b vdso: arch/arm64/kernel/vdso32/ arm's 32b vdso: arch/arm/kernel/vdso.c When you build a 32b kernel, you're only making use of the last of those three; the arm64 vdso and vdso32 code is irrelevant. This patch is specific to the second case, which is the 32b compat vdso for a 64b kernel. > > I'll drop this patch for now, while we figure it out a bit more. > > Cheers, > > Will -- Thanks, ~Nick Desaulniers
