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Date: Mon, 12 Dec 2016 06:48:27 +0100 From: "Jason A. Donenfeld" <Jason@...c4.com> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, LKML <linux-kernel@...r.kernel.org>, Linux Crypto Mailing List <linux-crypto@...r.kernel.org>, George Spelvin <linux@...izon.com>, Scott Bauer <sbauer@....utah.edu>, Andi Kleen <ak@...ux.intel.com>, Andy Lutomirski <luto@...capital.net>, Greg KH <gregkh@...uxfoundation.org>, Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>, "Daniel J . Bernstein" <djb@...yp.to> Subject: Re: [PATCH v2] siphash: add cryptographically secure hashtable function Hey Linus, On Mon, Dec 12, 2016 at 5:01 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote: > 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). I modified the test to hash data of size 0 through 7 repeatedly 100000000 times, and benchmarked that a few times on a Skylake laptop. The `load_unaligned_zeropad & bytemask_from_count` version was consistently 7% slower. I then modified it again to simply hash a 4 byte constant repeatedly 1000000000 times. The `load_unaligned_zeropad & bytemask_from_count` version was around 6% faster. I tried again with a 7 byte constant and got more or less a similar result. Then I tried with a 1 byte constant, and found that the `load_unaligned_zeropad & bytemask_from_count` version was slower. So, it would seem that between the `if (left)` and the `switch (left)`, there's the same number of branches. But for small values of `left`, the duff's device just has simpler arithmetic, whereas for large values of `left`, the `load_unaligned_zeropad` prevails. If micro-optimization is really appealing, one could imagine a hybrid of the two: switch (left) { case 7: case 6: case 5: case 4: b |= le64_to_cpu(load_unaligned_zeropad(data) & bytemask_from_count(left)); break; case 3: b |= ((u64)data[2]) << 16; case 2: b |= ((u64)data[1]) << 8; case 1: b |= ((u64)data[0]); break; case 0: break; } But I'm not sure this complication is worth it, and it might be more likely that the left-over size is 4 bytes most of the time, so we should just use your trick on platforms that support it. Jason
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