Re: [RFC V1 3/7] crypto: ghash - Optimized GHASH computations

[Ard Biesheuvel]

On Fri, 18 Dec 2020 at 22:07, Megha Dey <megha.dey@xxxxxxxxx> wrote:
>
> From: Kyung Min Park <kyung.min.park@xxxxxxxxx>
>
> Optimize GHASH computations with the 512 bit wide VPCLMULQDQ instructions.
> The new instruction allows to work on 4 x 16 byte blocks at the time.
> For best parallelism and deeper out of order execution, the main loop of
> the code works on 16 x 16 byte blocks at the time and performs reduction
> every 48 x 16 byte blocks. Such approach needs 48 precomputed GHASH subkeys
> and the precompute operation has been optimized as well to leverage 512 bit
> registers, parallel carry less multiply and reduction.
>
> VPCLMULQDQ instruction is used to accelerate the most time-consuming
> part of GHASH, carry-less multiplication. VPCLMULQDQ instruction
> with AVX-512F adds EVEX encoded 512 bit version of PCLMULQDQ instruction.
>
> The glue code in ghash_clmulni_intel module overrides existing PCLMULQDQ
> version with the VPCLMULQDQ version when the following criteria are met:
> At compile time:
> 1. CONFIG_CRYPTO_AVX512 is enabled
> 2. toolchain(assembler) supports VPCLMULQDQ instructions
> At runtime:
> 1. VPCLMULQDQ and AVX512VL features are supported on a platform (currently
>    only Icelake)
> 2. If compiled as built-in module, ghash_clmulni_intel.use_avx512 is set at
>    boot time or /sys/module/ghash_clmulni_intel/parameters/use_avx512 is set
>    to 1 after boot.
>    If compiled as loadable module, use_avx512 module parameter must be set:
>    modprobe ghash_clmulni_intel use_avx512=1
>
> With new implementation, tcrypt ghash speed test shows about 4x to 10x
> speedup improvement for GHASH calculation compared to the original
> implementation with PCLMULQDQ when the bytes per update size is 256 Bytes
> or above. Detailed results for a variety of block sizes and update
> sizes are in the table below. The test was performed on Icelake based
> platform with constant frequency set for CPU.
>
> The average performance improvement of the AVX512 version over the current
> implementation is as follows:
> For bytes per update >= 1KB, we see the average improvement of 882%(~8.8x).
> For bytes per update < 1KB, we see the average improvement of 370%(~3.7x).
>
> A typical run of tcrypt with GHASH calculation with PCLMULQDQ instruction
> and VPCLMULQDQ instruction shows the following results.
>
> ---------------------------------------------------------------------------
> |            |            |         cycles/operation         |            |
> |            |            |       (the lower the better)     |            |
> |    byte    |   bytes    |----------------------------------| percentage |
> |   blocks   | per update |   GHASH test   |   GHASH test    | loss/gain  |
> |            |            | with PCLMULQDQ | with VPCLMULQDQ |            |
> |------------|------------|----------------|-----------------|------------|
> |      16    |     16     |       144      |        233      |   -38.0    |
> |      64    |     16     |       535      |        709      |   -24.5    |
> |      64    |     64     |       210      |        146      |    43.8    |
> |     256    |     16     |      1808      |       1911      |    -5.4    |
> |     256    |     64     |       865      |        581      |    48.9    |
> |     256    |    256     |       682      |        170      |   301.0    |
> |    1024    |     16     |      6746      |       6935      |    -2.7    |
> |    1024    |    256     |      2829      |        714      |   296.0    |
> |    1024    |   1024     |      2543      |        341      |   645.0    |
> |    2048    |     16     |     13219      |      13403      |    -1.3    |
> |    2048    |    256     |      5435      |       1408      |   286.0    |
> |    2048    |   1024     |      5218      |        685      |   661.0    |
> |    2048    |   2048     |      5061      |        565      |   796.0    |
> |    4096    |     16     |     40793      |      27615      |    47.8    |
> |    4096    |    256     |     10662      |       2689      |   297.0    |
> |    4096    |   1024     |     10196      |       1333      |   665.0    |
> |    4096    |   4096     |     10049      |       1011      |   894.0    |
> |    8192    |     16     |     51672      |      54599      |    -5.3    |
> |    8192    |    256     |     21228      |       5284      |   301.0    |
> |    8192    |   1024     |     20306      |       2556      |   694.0    |
> |    8192    |   4096     |     20076      |       2044      |   882.0    |
> |    8192    |   8192     |     20071      |       2017      |   895.0    |
> ---------------------------------------------------------------------------
>
> This work was inspired by the AES GCM mode optimization published
> in Intel Optimized IPSEC Cryptographic library.
> https://github.com/intel/intel-ipsec-mb/lib/avx512/gcm_vaes_avx512.asm
>
> Co-developed-by: Greg Tucker <greg.b.tucker@xxxxxxxxx>
> Signed-off-by: Greg Tucker <greg.b.tucker@xxxxxxxxx>
> Co-developed-by: Tomasz Kantecki <tomasz.kantecki@xxxxxxxxx>
> Signed-off-by: Tomasz Kantecki <tomasz.kantecki@xxxxxxxxx>
> Signed-off-by: Kyung Min Park <kyung.min.park@xxxxxxxxx>
> Co-developed-by: Megha Dey <megha.dey@xxxxxxxxx>
> Signed-off-by: Megha Dey <megha.dey@xxxxxxxxx>

Hello Megha,

What is the purpose of this separate GHASH module? GHASH is only used
in combination with AES-CTR to produce GCM, and this series already
contains a GCM driver.

Do cores exist that implement PCLMULQDQ but not AES-NI?

If not, I think we should be able to drop this patch (and remove the
existing PCLMULQDQ GHASH driver as well)