AES-NI: slower than aes-generic?

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Hi,

for the DRBG and the LRNG work I am doing, I also test the speed of the DRBG. 
The DRBG can be considered as a form of block chaining mode on top of a raw 
cipher.

What I am wondering is that when encrypting 256 16 byte blocks, I get a speed 
of about 170 MB/s with the AES-NI driver. When using the aes-generic or aes-
asm, I get up to 180 MB/s with all else being equal. Note, that figure 
includes a copy_to_user of the generated data.

To be precise, the code does the following steps:

1. setkey

2. AES encrypt 256 blocks and copy_to_user

3. Use AES to generate a new key and start at step 1.

I am wondering why the AES-NI driver is slower than the C/ASM implementations 
given that all else is equal.

Note, if I have less blocks in step 2 above, AES-NI is becoming faster. E.g. 
if I have 8 blocks or just one block in step 2 above, AES-NI is faster by 10 
or 20%.

Ciao
Stephan
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