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Re: [PATCH v2] siphash: add cryptographically secure hashtable function

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On Sun, Dec 11, 2016 at 7:48 PM, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> +       switch (left) {
> +               case 7: b |= ((u64)data[6]) << 48;
> +               case 6: b |= ((u64)data[5]) << 40;
> +               case 5: b |= ((u64)data[4]) << 32;
> +               case 4: b |= ((u64)data[3]) << 24;
> +               case 3: b |= ((u64)data[2]) << 16;
> +               case 2: b |= ((u64)data[1]) <<  8;
> +               case 1: b |= ((u64)data[0]); break;
> +               case 0: break;
> +       }

The above is extremely inefficient. Considering that most kernel data
would be expected to be smallish, that matters (ie the usual benchmark
would not be about hashing megabytes of data, but instead millions of
hashes of small data).

I think this could be rewritten (at least for 64-bit architectures) as

    #ifdef CONFIG_DCACHE_WORD_ACCESS

        if (left)
                b |= le64_to_cpu(load_unaligned_zeropad(data) &
bytemask_from_count(left));

    #else

        .. do the duff's device thing with the switch() ..

    #endif

which should give you basically perfect code generation (ie a single
64-bit load and a byte mask).

Totally untested, just looking at the code and trying to make sense of it.

... and obviously, it requires an actual high-performance use-case to
make any difference.

                  Linus

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[PATCH] siphash: add cryptographically secure hashtable function "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-09 19:40 +0100
  Re: [kernel-hardening] [PATCH] siphash: add cryptographically secure  hashtable function Greg KH <gregkh@linuxfoundation.org> - 2016-12-10 13:40 +0100
    Re: [kernel-hardening] [PATCH] siphash: add cryptographically secure  hashtable function "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-11 16:40 +0100
      Re: [kernel-hardening] [PATCH] siphash: add cryptographically secure  hashtable function Greg KH <gregkh@linuxfoundation.org> - 2016-12-11 21:50 +0100
        [PATCH v2] siphash: add cryptographically secure hashtable function "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-12 04:50 +0100
          Re: [PATCH v2] siphash: add cryptographically secure hashtable function Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-12 05:10 +0100
            Re: [PATCH v2] siphash: add cryptographically secure hashtable function "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-12 06:50 +0100
              Re: [PATCH v2] siphash: add cryptographically secure hashtable function Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-12 22:40 +0100
                [PATCH v3] siphash: add cryptographically secure hashtable function "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-12 23:30 +0100
                Re: [PATCH v3] siphash: add cryptographically secure hashtable  function Andi Kleen <ak@linux.intel.com> - 2016-12-13 00:10 +0100
                Re: [PATCH v3] siphash: add cryptographically secure hashtable  function Eric Biggers <ebiggers3@gmail.com> - 2016-12-13 09:50 +0100
                Re: [PATCH v3] siphash: add cryptographically secure hashtable function Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-13 20:30 +0100
                Re: [PATCH v3] siphash: add cryptographically secure hashtable function "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-13 23:50 +0100
                [PATCH v4] siphash: add cryptographically secure hashtable function "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-13 23:50 +0100
          Re: [PATCH v2] siphash: add cryptographically secure hashtable  function Eric Biggers <ebiggers3@gmail.com> - 2016-12-12 06:50 +0100
            Re: [PATCH v2] siphash: add cryptographically secure hashtable function "Jason A. Donenfeld" <Jason@zx2c4.com> - 2016-12-12 22:20 +0100
  Re: [PATCH] siphash: add cryptographically secure hashtable function Vegard Nossum <vegard.nossum@gmail.com> - 2016-12-10 15:20 +0100
    Re: [PATCH] siphash: add cryptographically secure hashtable function "George Spelvin" <linux@sciencehorizons.net> - 2016-12-10 16:50 +0100

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