Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc

[hpa@zytor.com]

On March 15, 2019 4:34:10 PM PDT, Matthias Kaehlcke <mka@chromium.org> wrote:
>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)?

Yes.
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