Am 13.03.2012 19:58, schrieb Florian Philipp: > Am 13.03.2012 19:18, schrieb Michael Mol: >> On Tue, Mar 13, 2012 at 2:06 PM, Florian Philipp <li...@binarywings.net> >> wrote: >>> Am 13.03.2012 18:45, schrieb Frank Steinmetzger: >>>> On Tue, Mar 13, 2012 at 05:11:47PM +0100, Florian Philipp wrote: >>>> >>>>>> Since I am planning to encrypt only home/ under LVM control, what kind >>>>>> of overhead should I expect? >>>>> >>>>> What do you mean with overhead? CPU utilization? In that case the >>>>> overhead is minimal, especially when you run a 64-bit kernel with the >>>>> optimized AES kernel module. >>>> >>>> Speaking of that... >>>> I always wondered what the exact difference was between AES and AES i586. I >>>> can gather myself that it's about optimisation for a specific architecture. >>>> But which one would be best for my i686 Core 2 Duo? >>> >>> From what I can see in the kernel sources, there is a generic AES >>> implementation using nothing but portable C code and then there is >>> "aes-i586" assembler code with "aes_glue" C code. >> >> >>> So I assume the i586 >>> version is better for you --- unless GCC suddenly got a lot better at >>> optimizing code. >> >> Since when, exactly? GCC isn't the best compiler at optimization, but >> I fully expect current versions to produce better code for x86-64 than >> hand-tuned i586. Wider registers, more registers, crypto acceleration >> instructions and SIMD instructions are all very nice to have. I don't >> know the specifics of AES, though, or what kind of crypto algorithm it >> is, so it's entirely possible that one can't effectively parallelize >> it except in some relatively unique circumstances. >> > > One sec. We are talking about an Core2 Duo running in 32bit mode, right? > That's what the i686 reference in the question meant --- or at least, > that's what I assumed. > > If we talk about 32bit mode, none of what you describe is available. > Those additional registers and instructions are not accessible with i686 > instructions. A Core 2 also has no AES instructions. > > Of course, GCC could make use of what it knows about the CPU, like > number of parallel pipelines, pipeline depth, cache size, instructions > added in i686 and so on. But even then I doubt it can outperform > hand-tuned assembler, even if it is for a slightly older instruction set. >
P.S: I just looked up the differences in the instruction sets of i586 and i686. The only significant instruction added in i686 was a conditional move (CMOV). This helps to avoid condition jumps. However, in the aes-i586 code there are only two conditional jumps and they both just end the loop of encryption/decryption rounds for AES-128 and AES256, respectively. My assembler isn't perfect but I doubt you can optimize that away with a CMOV. > If instead we are talking about an Core 2 Duo running in x86_64 mode, we > should be talking about the aes-x86_64 module instead of the aes-i586 > module and that makes use of the complete instruction set of the Core 2, > including SSE2. > > Regards, > Florian Philipp
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