Re: [PATCH 5/7] random: replace non-blocking pool with a Chacha20-based CRNG

[Herbert Xu <herbert@gondor.apana.org.au>]

On Mon, Jun 20, 2016 at 01:02:03AM -0400, Theodore Ts'o wrote:
> 
> It's work that I'm not convinced is worth the gain?  Perhaps I
> shouldn't have buried the lede, but repeating a paragraph from later
> in the message:
> 
>    So even if the AVX optimized is 100% faster than the generic version,
>    it would change the time needed to create a 256 byte session key from
>    1.68 microseconds to 1.55 microseconds.  And this is ignoring the
>    extra overhead needed to set up AVX, the fact that this will require
>    the kernel to do extra work doing the XSAVE and XRESTORE because of
>    the use of the AVX registers, etc.

We do have figures on the efficiency of the accelerated chacha
implementation on 256-byte requests (I've picked the 8-block
version):

testing speed of chacha20 (chacha20-generic) encryption
test 2 (256 bit key, 256 byte blocks): 12702056 operations in 10 seconds (3251726336 bytes)

testing speed of chacha20 (chacha20-simd) encryption
test 2 (256 bit key, 256 byte blocks): 33028112 operations in 10 seconds (8455196672 bytes)

So it is a little bit more than 100%.

> So in the absolute best case, this improves the time needed to create
> a 256 bit session key by 0.13 microseconds.  And that assumes that the
> extra setup and teardown overhead of an AVX optimized ChaCha20
> (including the XSAVE and XRESTORE of the AVX registers, etc.) don't
> end up making the CRNG **slower**.

The figures above include all of these overheads.  The overheads
really only show up on 16-byte requests.
 
> P.S.  I haven't measured this to see, mainly because I really don't
> care about the difference between 1.68 vs 1.55 microseconds, but there
> is a good chance in the crypto layer that it might be a good idea to
> have the system be smart enough to automatically fall back to using
> the **non** optimized version if you only need to encrypt a small
> amount of data.

You're right.  chacha20-simd should use the generic version on
16-byte requests which is the only place where it is slower.
Something like this:

---8<---
Subject: crypto: chacha20-simd - Use generic code for small requests

On 16-byte requests the optimised version is actually slower than
the generic code, so we should simply use that instead.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/arch/x86/crypto/chacha20_glue.c b/arch/x86/crypto/chacha20_glue.c
index 2d5c2e0b..f910d1d 100644
--- a/arch/x86/crypto/chacha20_glue.c
+++ b/arch/x86/crypto/chacha20_glue.c
@@ -70,7 +70,7 @@ static int chacha20_simd(struct blkcipher_desc *desc, struct scatterlist *dst,
 	struct blkcipher_walk walk;
 	int err;
 
-	if (!may_use_simd())
+	if (nbytes <= CHACHA20_BLOCK_SIZE || !may_use_simd())
 		return crypto_chacha20_crypt(desc, dst, src, nbytes);
 
 	state = (u32 *)roundup((uintptr_t)state_buf, CHACHA20_STATE_ALIGN);

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt