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e6e758fa64
Rewrite the AES-NI implementations of AES-GCM, taking advantage of
things I learned while writing the VAES-AVX10 implementations. This is
a complete rewrite that reduces the AES-NI GCM source code size by about
70% and the binary code size by about 95%, while not regressing
performance and in fact improving it significantly in many cases.
The following summarizes the state before this patch:
- The aesni-intel module registered algorithms "generic-gcm-aesni" and
"rfc4106-gcm-aesni" with the crypto API that actually delegated to one
of three underlying implementations according to the CPU capabilities
detected at runtime: AES-NI, AES-NI + AVX, or AES-NI + AVX2.
- The AES-NI + AVX and AES-NI + AVX2 assembly code was in
aesni-intel_avx-x86_64.S and consisted of 2804 lines of source and
257 KB of binary. This massive binary size was not really
appropriate, and depending on the kconfig it could take up over 1% the
size of the entire vmlinux. The main loops did 8 blocks per
iteration. The AVX code minimized the use of carryless multiplication
whereas the AVX2 code did not. The "AVX2" code did not actually use
AVX2; the check for AVX2 was really a check for Intel Haswell or later
to detect support for fast carryless multiplication. The long source
length was caused by factors such as significant code duplication.
- The AES-NI only assembly code was in aesni-intel_asm.S and consisted
of 1501 lines of source and 15 KB of binary. The main loops did 4
blocks per iteration and minimized the use of carryless multiplication
by using Karatsuba multiplication and a multiplication-less reduction.
- The assembly code was contributed in 2010-2013. Maintenance has been
sporadic and most design choices haven't been revisited.
- The assembly function prototypes and the corresponding glue code were
separate from and were not consistent with the new VAES-AVX10 code I
recently added. The older code had several issues such as not
precomputing the GHASH key powers, which hurt performance.
This rewrite achieves the following goals:
- Much shorter source and binary sizes. The assembly source shrinks
from 4300 lines to 1130 lines, and it produces about 9 KB of binary
instead of 272 KB. This is achieved via a better designed AES-GCM
implementation that doesn't excessively unroll the code and instead
prioritizes the parts that really matter. Sharing the C glue code
with the VAES-AVX10 implementations also saves 250 lines of C source.
- Improve performance on most (possibly all) CPUs on which this code
runs, for most (possibly all) message lengths. Benchmark results are
given in Tables 1 and 2 below.
- Use the same function prototypes and glue code as the new VAES-AVX10
algorithms. This fixes some issues with the integration of the
assembly and results in some significant performance improvements,
primarily on short messages. Also, the AVX and non-AVX
implementations are now registered as separate algorithms with the
crypto API, which makes them both testable by the self-tests.
- Keep support for AES-NI without AVX (for Westmere, Silvermont,
Goldmont, and Tremont), but unify the source code with AES-NI + AVX.
Since 256-bit vectors cannot be used without VAES anyway, this is made
feasible by just using the non-VEX coded form of most instructions.
- Use a unified approach where the main loop does 8 blocks per iteration
and uses Karatsuba multiplication to save one pclmulqdq per block but
does not use the multiplication-less reduction. This strikes a good
balance across the range of CPUs on which this code runs.
- Don't spam the kernel log with an informational message on every boot.
The following tables summarize the improvement in AES-GCM throughput on
various CPU microarchitectures as a result of this patch:
Table 1: AES-256-GCM encryption throughput improvement,
CPU microarchitecture vs. message length in bytes:
| 16384 | 4096 | 4095 | 1420 | 512 | 500 |
-------------------+-------+-------+-------+-------+-------+-------+
Intel Broadwell | 2% | 8% | 11% | 18% | 31% | 26% |
Intel Skylake | 1% | 4% | 7% | 12% | 26% | 19% |
Intel Cascade Lake | 3% | 8% | 10% | 18% | 33% | 24% |
AMD Zen 1 | 6% | 12% | 6% | 15% | 27% | 24% |
AMD Zen 2 | 8% | 13% | 13% | 19% | 26% | 28% |
AMD Zen 3 | 8% | 14% | 13% | 19% | 26% | 25% |
| 300 | 200 | 64 | 63 | 16 |
-------------------+-------+-------+-------+-------+-------+
Intel Broadwell | 35% | 29% | 45% | 55% | 54% |
Intel Skylake | 25% | 19% | 28% | 33% | 27% |
Intel Cascade Lake | 36% | 28% | 39% | 49% | 54% |
AMD Zen 1 | 27% | 22% | 23% | 29% | 26% |
AMD Zen 2 | 32% | 24% | 22% | 25% | 31% |
AMD Zen 3 | 30% | 24% | 22% | 23% | 26% |
Table 2: AES-256-GCM decryption throughput improvement,
CPU microarchitecture vs. message length in bytes:
| 16384 | 4096 | 4095 | 1420 | 512 | 500 |
-------------------+-------+-------+-------+-------+-------+-------+
Intel Broadwell | 3% | 8% | 11% | 19% | 32% | 28% |
Intel Skylake | 3% | 4% | 7% | 13% | 28% | 27% |
Intel Cascade Lake | 3% | 9% | 11% | 19% | 33% | 28% |
AMD Zen 1 | 15% | 18% | 14% | 20% | 36% | 33% |
AMD Zen 2 | 9% | 16% | 13% | 21% | 26% | 27% |
AMD Zen 3 | 8% | 15% | 12% | 18% | 23% | 23% |
| 300 | 200 | 64 | 63 | 16 |
-------------------+-------+-------+-------+-------+-------+
Intel Broadwell | 36% | 31% | 40% | 51% | 53% |
Intel Skylake | 28% | 21% | 23% | 30% | 30% |
Intel Cascade Lake | 36% | 29% | 36% | 47% | 53% |
AMD Zen 1 | 35% | 31% | 32% | 35% | 36% |
AMD Zen 2 | 31% | 30% | 27% | 38% | 30% |
AMD Zen 3 | 27% | 23% | 24% | 32% | 26% |
The above numbers are percentage improvements in single-thread
throughput, so e.g. an increase from 3000 MB/s to 3300 MB/s would be
listed as 10%. They were collected by directly measuring the Linux
crypto API performance using a custom kernel module. Note that indirect
benchmarks (e.g. 'cryptsetup benchmark' or benchmarking dm-crypt I/O)
include more overhead and won't see quite as much of a difference. All
these benchmarks used an associated data length of 16 bytes. Note that
AES-GCM is almost always used with short associated data lengths.
I didn't test Intel CPUs before Broadwell, AMD CPUs before Zen 1, or
Intel low-power CPUs, as these weren't readily available to me.
However, based on the design of the new code and the available
information about these other CPU microarchitectures, I wouldn't expect
any significant regressions, and there's a good chance performance is
improved just as it is above.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.