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

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 [off-list ref]
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt