Messages in this thread | | | Date | Mon, 11 Jun 2007 15:45:36 -0400 | From | Benjamin Gilbert <> | Subject | Re: [PATCH 3/3] [CRYPTO] Add optimized SHA-1 implementation for x86_64 |
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Andi Kleen wrote: > Benjamin Gilbert <bgilbert@cs.cmu.edu> writes: >> +#define EXPAND(i) \ >> + movl OFFSET(i % 16)(DATA), TMP; \ >> + xorl OFFSET((i + 2) % 16)(DATA), TMP; \ > > Such overlapping memory accesses are somewhat dangerous as they tend > to stall some CPUs. Better probably to do a quad load and then extract.
OFFSET(i) is defined as 4*(i), so they don't actually overlap. (Arguably that macro should go away.)
> I haven't checked in detail if it's possible but it's suspicious you > never use quad operations for anything. You keep at least half > the CPU's bits idle all the time.
SHA-1 fundamentally wants to work with 32-bit quantities. It might be possible to use quad operations for some things, with sufficient cleverness, but I doubt it'd be worth the effort.
> Gut feeling is that the unroll factor is far too large. > Have you tried a smaller one? That would save icache > which is very important in the kernel.
That seems to be the consensus. I'll see if I can find some time to try linux@horizon.com's suggestion and report back.
I don't think, though, that cache footprint is the *only* thing that matters. Leaving aside /dev/urandom, there are cases where throughput matters a lot. This patch set came out of some work on a hashing block device driver in which SHA is, by far, the biggest CPU user. One could imagine content-addressable filesystems, or even IPsec under the right workloads, being in a similar situation.
Would it be more palatable to roll the patch as an optimized CryptoAPI module rather than as a lib/sha1.c replacement? That wouldn't help /dev/urandom, of course, but for other cases it would allow the user to ask for the optimized version if needed, and not pay the footprint costs otherwise.
--Benjamin Gilbert
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