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Date:   Mon, 18 Mar 2019 16:54:25 -0700
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Subject: Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc
To:     Matthias Kaehlcke <mka@chromium.org>
CC:     Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        Manoj Gupta <manojgupta@chromium.org>,
        Tiancong Wang <tcwang@chromium.org>,
        Stephen Hines <srhines@google.com>,
        clang-built-linux@googlegroups.com
From:   hpa@zytor.com
Message-ID: <8F53628B-877F-4FA9-B225-FF4135147B46@zytor.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On March 18, 2019 4:52:19 PM PDT, Matthias Kaehlcke <mka@chromium.org> wrote:
>On Mon, Mar 18, 2019 at 04:44:03PM -0700, hpa@zytor.com wrote:
>> On March 18, 2019 3:16:39 PM PDT, Matthias Kaehlcke
><mka@chromium.org> wrote:
>> >On Mon, Mar 18, 2019 at 02:50:44PM -0700, hpa@zytor.com wrote:
>> >> On March 18, 2019 2:31:13 PM PDT, Matthias Kaehlcke
>> ><mka@chromium.org> wrote:
>> >> >On Fri, Mar 15, 2019 at 01:54:50PM -0700, Matthias Kaehlcke
>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'
>> >> >> 
>> >> >> Add a copy of the __lshrti3 libgcc routine (from gcc v4.9.2).
>> >> >> 
>> >> >> 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>
>> >> >
>> >> >With "Revert "kbuild: use -Oz instead of -Os when using clang"
>> >> >(https://lore.kernel.org/patchwork/patch/1051932/) the above
>> >> >error is fixed, a few comments inline for if the patch is
>> >> >resurrected in the future because __lshrti3 is emitted in a
>> >> >different context.
>> >> >
>> >> >> 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)));
>> >> >
>> >> >Consider using __int128 instead. Definition and use need a
>> >> >'defined(__SIZEOF_INT128__)' guard  (similar for mode (TI)),
>since
>> >> >these 128 bit types aren't supported on all platforms.
>> >> >
>> >> >>  #ifdef __BIG_ENDIAN
>> >> >>  struct DWstruct {
>> >> >>  	int high, low;
>> >> >>  };
>> >> >> +
>> >> >> +struct DWstruct128 {
>> >> >> +	long long high, low;
>> >> >> +};
>> >> >
>> >> >This struct isn't needed, struct DWstruct can be used.
>> >> >
>> >> >> 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)
>> >> >
>> >> >use TItype for input/output, which is what gcc does, though the
>> >above
>> >> >matches the interface in the documentation.
>> >> >
>> >> >> +{
>> >> >> +	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;
>> >> >
>> >> >include <linux/types.h> and use u64 instead of unsigned long
>long.
>> >> 
>> >> Ok, now I'm really puzzled.
>> >> 
>> >> How could we need a 128-bit shift when the prototype only has 64
>bits
>> >of input?!
>> >
>> >Good question, this is the code from libgcc:
>> >
>> >TItype
>> >__lshrti3 (TItype u, shift_count_type b)
>> >{
>> >  if (b == 0)
>> >    return u;
>> >
>> >  const DWunion uu = {.ll = u};
>> >  const shift_count_type bm = (8 * (8)) - b;
>> >  DWunion w;
>> >
>> >  if (bm <= 0)
>> >    {
>> >      w.s.high = 0;
>> >      w.s.low = (UDItype) uu.s.high >> -bm;
>> >    }
>> >  else
>> >    {
>> >      const UDItype carries = (UDItype) uu.s.high << bm;
>> >
>> >      w.s.high = (UDItype) uu.s.high >> b;
>> >      w.s.low = ((UDItype) uu.s.low >> b) | carries;
>> >    }
>> >
>> >  return w.ll;
>> >}
>> >
>> >
>> >My compiler knowledge is limited, my guess is that the function is a
>> >generic implementation, and while a long long is 64-bit wide under
>> >Linux it could be 128-bit on other platforms.
>> 
>> Yes, long long is just plain wrong.
>> 
>> How could we end up calling this function on 32 bits?!
>
>We didn't, in this case the function is called in 64-bit code
>(arch/x86/kvm/x86.o: In function `mul_u64_u64_shr'), for the 32-bit
>vDSO it was __lshrdi3.

That makes more sense.
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