On Tue, Aug 4, 2020 at 2:09 AM 'Nick Desaulniers' via Clang Built Linux <clang-built-li...@googlegroups.com> wrote: > > On Wed, May 27, 2020 at 6:53 AM Arnd Bergmann <a...@arndb.de> wrote: > > > > When using the clang integrated assembler, we get a reference > > to __force_order that should normally get ignored in a few > > rare cases: > > > > ERROR: modpost: "__force_order" [drivers/cpufreq/powernow-k6.ko] undefined! > > > > Add a 'static' definition so any file in which this happens can > > have a local copy. > > > > Signed-off-by: Arnd Bergmann <a...@arndb.de> > > Hi Arnd, > Looks like > $ ARCH=i386 make CC=clang LLVM_IAS=1 -j71 > defconfig+CONFIG_X86_POWERNOW_K6=m > is the simplest reproducer. > > If I run > $ llvm-readelf -s drivers/cpufreq/powernow-k6.o | grep __force_order > 39: 00000000 0 NOTYPE GLOBAL DEFAULT UND __force_order > we can indeed see an undefined reference to __force_order. > > If I build the .s file via > $ ARCH=i386 make CC=clang LLVM_IAS=1 -j71 drivers/cpufreq/powernow-k6.s > the only reference I see to __force_order is: > 979 .addrsig_sym __force_order > > which is created by Clang's implicit -faddr-sig. If I disable that > for this file via: > > ```diff > diff --git a/drivers/cpufreq/Makefile b/drivers/cpufreq/Makefile > index f1b7e3dd6e5d..87d655d5af49 100644 > --- a/drivers/cpufreq/Makefile > +++ b/drivers/cpufreq/Makefile > @@ -28,6 +28,9 @@ obj-$(CONFIG_X86_ACPI_CPUFREQ) += > acpi-cpufreq.o > obj-$(CONFIG_X86_POWERNOW_K8) += powernow-k8.o > obj-$(CONFIG_X86_PCC_CPUFREQ) += pcc-cpufreq.o > obj-$(CONFIG_X86_POWERNOW_K6) += powernow-k6.o > +ifdef CONFIG_CC_IS_CLANG > +CFLAGS_powernow-k6.o += -fno-addrsig > +endif > obj-$(CONFIG_X86_POWERNOW_K7) += powernow-k7.o > obj-$(CONFIG_X86_LONGHAUL) += longhaul.o > obj-$(CONFIG_X86_E_POWERSAVER) += e_powersaver.o > ``` > then the module links without error, and we get no hits for > __force_order from llvm-readelf -s. This makes me think there may be > a bug in Clang generating address significance tables for global > variables that are otherwise unused, resulting in such linkage > failures. +pcc@ for that. > > I ran a creduce job on drivers/cpufreq/powernow-k6.i where I'd compile > twice, one with the implicit default value of -faddr-sig and look for > the undefined __force_order, and again with -fno-addrsig and ensure > there was no undefined __force_order, which coughed up: > extern int __force_order; > int a(void) { asm("" : "=m"(__force_order)); return 0; } > as the bare minimum for an address significant table. > https://godbolt.org/z/cjfaqM > > I'll bet this is coming from the call to read_cr0() in > powernow_k6_set_cpu_multiplier(). If __force_order is defined in > arch/x86/boot/compressed/pgtable_64.c, then I'm not sure it's a good > idea to build drivers/cpufreq/powernow-k6.c as a kernel module > (CONFIG_X86_POWERNOW_K6=m) vs statically compiled in. Wouldn't > __force_order need to be EXPORT'ed for kernel modules to use it > safely? > > > --- > > arch/x86/boot/compressed/pgtable_64.c | 2 ++ > > arch/x86/include/asm/special_insns.h | 7 +++++++ > > 2 files changed, 9 insertions(+) > > > > diff --git a/arch/x86/boot/compressed/pgtable_64.c > > b/arch/x86/boot/compressed/pgtable_64.c > > index c8862696a47b..8595194cea41 100644 > > --- a/arch/x86/boot/compressed/pgtable_64.c > > +++ b/arch/x86/boot/compressed/pgtable_64.c > > @@ -12,7 +12,9 @@ > > * It is not referenced from the code, but GCC < 5 with -fPIE would fail > > * due to an undefined symbol. Define it to make these ancient GCCs work. > > */ > > +#ifndef CONFIG_CC_IS_CLANG > > unsigned long __force_order; > > +#endif > > > > #define BIOS_START_MIN 0x20000U /* 128K, less than this is > > insane */ > > #define BIOS_START_MAX 0x9f000U /* 640K, absolute maximum */ > > diff --git a/arch/x86/include/asm/special_insns.h > > b/arch/x86/include/asm/special_insns.h > > index 82436cb04ccf..7081e587c1ea 100644 > > --- a/arch/x86/include/asm/special_insns.h > > +++ b/arch/x86/include/asm/special_insns.h > > @@ -16,8 +16,15 @@ > > * A memory clobber would solve the problem, but would prevent reordering > > of > > * all loads stores around it, which can hurt performance. Solution is to > > * use a variable and mimic reads and writes to it to enforce serialization > > + * > > + * Clang sometimes fails to kill the reference to the dummy variable, so > > + * provide an actual copy. > > */ > > +#ifdef CONFIG_CC_IS_CLANG > > +static unsigned long __force_order; > > +#else > > extern unsigned long __force_order; > > +#endif > > > > void native_write_cr0(unsigned long val); > > > > -- > > 2.26.2 > >
Thanks for the proposal. I have adapted it to fit my patchset against Linux v5.8. Both Debian's GCC-10 and a snapshot version of LLVM toolchain v11.0.0-rc1+ seems to be OK. MAKE_OPTS="V=1 -j3 CC=gcc-10 LD=ld.bfd" make $MAKE_OPTS arch/x86/kernel/cpu/common.o MAKE_OPTS="V=1 -j3 HOSTCC=clang-11 HOSTCXX=clang++-11 HOSTLD=ld.lld-11 HOSTAR=llvm-ar-11 CC=clang-11 LD=ld.lld-11 AR=llvm-ar-11 NM=llvm-nm-11 OBJCOPY=llvm-objcopy-11 OBJDUMP=llvm-objdump-11 OBJSIZE=llvm-size-11 READELF=llvm-readelf-11 STRIP=llvm-strip-11 LLVM_IAS=1" make $MAKE_OPTS arch/x86/kernel/cpu/common.o I can send both object files if desired. I will do a full kernel-build to see if I am able to build the VirtualBox out-of-tree kernel-modules. - Sedat -