Messages in this thread | | | Date | Fri, 15 Mar 2019 16:34:10 -0700 | From | Matthias Kaehlcke <> | Subject | Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc |
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Hi Peter,
On Fri, Mar 15, 2019 at 03:08:57PM -0700, hpa@zytor.com wrote: > On March 15, 2019 3:06:37 PM PDT, Nick Desaulniers <ndesaulniers@google.com> wrote: > >On Fri, Mar 15, 2019 at 1:54 PM Matthias Kaehlcke <mka@chromium.org> > >wrote: > >> > >> The compiler may emit calls to __lshrti3 from the compiler runtime > >> library, which results in undefined references: > >> > >> arch/x86/kvm/x86.o: In function `mul_u64_u64_shr': > >> include/linux/math64.h:186: undefined reference to `__lshrti3' > > > >Looks like Clang will emit this at -Oz (but not -O2): > >https://godbolt.org/z/w1_2YC > > > >> > >> Add a copy of the __lshrti3 libgcc routine (from gcc v4.9.2). > > > >Has it changed since? If so why not a newer version of libgcc_s? > > > >> > >> Include the function for x86 builds with clang, which is the > >> environment where the above error was observed. > >> > >> Signed-off-by: Matthias Kaehlcke <mka@chromium.org> > >> --- > >> arch/x86/Kconfig | 1 + > >> include/linux/libgcc.h | 16 ++++++++++++++++ > >> lib/Kconfig | 3 +++ > >> lib/Makefile | 1 + > >> lib/lshrti3.c | 31 +++++++++++++++++++++++++++++++ > >> 5 files changed, 52 insertions(+) > >> create mode 100644 lib/lshrti3.c > >> > >> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig > >> index c1f9b3cf437c..a5e0d923845d 100644 > >> --- a/arch/x86/Kconfig > >> +++ b/arch/x86/Kconfig > >> @@ -105,6 +105,7 @@ config X86 > >> select GENERIC_IRQ_PROBE > >> select GENERIC_IRQ_RESERVATION_MODE > >> select GENERIC_IRQ_SHOW > >> + select GENERIC_LIB_LSHRTI3 if CC_IS_CLANG > >> select GENERIC_PENDING_IRQ if SMP > >> select GENERIC_SMP_IDLE_THREAD > >> select GENERIC_STRNCPY_FROM_USER > >> diff --git a/include/linux/libgcc.h b/include/linux/libgcc.h > >> index 32e1e0f4b2d0..a71036471838 100644 > >> --- a/include/linux/libgcc.h > >> +++ b/include/linux/libgcc.h > >> @@ -22,15 +22,26 @@ > >> #include <asm/byteorder.h> > >> > >> typedef int word_type __attribute__ ((mode (__word__))); > >> +typedef int TItype __attribute__ ((mode (TI))); > > > >Well that's an interesting new compiler attribute. > >https://stackoverflow.com/questions/4559025/what-does-gcc-attribute-modexx-actually-do > >Please use `__mode(TI)` from include/linux/compiler_attributes.h ex. > >typedef int TItype __mode(TI); > > > >> > >> #ifdef __BIG_ENDIAN > >> struct DWstruct { > >> int high, low; > >> }; > >> + > >> +struct DWstruct128 { > >> + long long high, low; > >> +}; > >> + > >> #elif defined(__LITTLE_ENDIAN) > >> struct DWstruct { > >> int low, high; > >> }; > >> + > >> +struct DWstruct128 { > >> + long long low, high; > >> +}; > >> + > >> #else > >> #error I feel sick. > >> #endif > >> @@ -40,4 +51,9 @@ typedef union { > >> long long ll; > >> } DWunion; > >> > >> +typedef union { > >> + struct DWstruct128 s; > >> + TItype ll; > >> +} DWunion128; > >> + > >> #endif /* __ASM_LIBGCC_H */ > >> diff --git a/lib/Kconfig b/lib/Kconfig > >> index a9e56539bd11..369e10259ea6 100644 > >> --- a/lib/Kconfig > >> +++ b/lib/Kconfig > >> @@ -624,6 +624,9 @@ config GENERIC_LIB_ASHRDI3 > >> config GENERIC_LIB_LSHRDI3 > >> bool > >> > >> +config GENERIC_LIB_LSHRTI3 > >> + bool > >> + > >> config GENERIC_LIB_MULDI3 > >> bool > >> > >> diff --git a/lib/Makefile b/lib/Makefile > >> index 4e066120a0d6..42648411f451 100644 > >> --- a/lib/Makefile > >> +++ b/lib/Makefile > >> @@ -277,6 +277,7 @@ obj-$(CONFIG_PARMAN) += parman.o > >> obj-$(CONFIG_GENERIC_LIB_ASHLDI3) += ashldi3.o > >> obj-$(CONFIG_GENERIC_LIB_ASHRDI3) += ashrdi3.o > >> obj-$(CONFIG_GENERIC_LIB_LSHRDI3) += lshrdi3.o > >> +obj-$(CONFIG_GENERIC_LIB_LSHRTI3) += lshrti3.o > >> obj-$(CONFIG_GENERIC_LIB_MULDI3) += muldi3.o > >> obj-$(CONFIG_GENERIC_LIB_CMPDI2) += cmpdi2.o > >> obj-$(CONFIG_GENERIC_LIB_UCMPDI2) += ucmpdi2.o > >> diff --git a/lib/lshrti3.c b/lib/lshrti3.c > >> new file mode 100644 > >> index 000000000000..2d2123bb3030 > >> --- /dev/null > >> +++ b/lib/lshrti3.c > >> @@ -0,0 +1,31 @@ > >> +// SPDX-License-Identifier: GPL-2.0 > >> + > >> +#include <linux/export.h> > >> +#include <linux/libgcc.h> > >> + > >> +long long __lshrti3(long long u, word_type b) > >> +{ > >> + DWunion128 uu, w; > >> + word_type bm; > >> + > >> + if (b == 0) > >> + return u; > >> + > >> + uu.ll = u; > >> + bm = 64 - b; > >> + > >> + if (bm <= 0) { > >> + w.s.high = 0; > >> + w.s.low = (unsigned long long) uu.s.high >> -bm; > >> + } else { > >> + const unsigned long long carries = > >> + (unsigned long long) uu.s.high << bm; > >> + w.s.high = (unsigned long long) uu.s.high >> b; > >> + w.s.low = ((unsigned long long) uu.s.low >> b) | > >carries; > >> + } > >> + > >> + return w.ll; > >> +} > >> +#ifndef BUILD_VDSO > >> +EXPORT_SYMBOL(__lshrti3); > >> +#endif > > > >I don't think you want this. Maybe that was carried over from > >https://lkml.org/lkml/2019/3/15/669 > >by accident? The above linkage error mentions arch/x86/kvm/x86.o > >which I wouldn't expect to be linked into the VDSO image? > > If we compile with the default ABI we can simply link with libgcc; > with -mregparm=3 all versions of gcc are broken.
I suppose you refer to linking the VDSO against libgcc? Would this be acceptable (as opposed to linking the kernel against it)?
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