linux-kernel - [PATCH 0/2] riscv: enable EFFICIENT_UNALIGNED_ACCESS and DCACHE_WORD

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Message-Id: <20231202111822.3569-1-jszhang@kernel.org>
Date:   Sat,  2 Dec 2023 19:18:20 +0800
From:   Jisheng Zhang <jszhang@...nel.org>
To:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH 0/2] riscv: enable EFFICIENT_UNALIGNED_ACCESS and DCACHE_WORD_ACCESS

Some riscv implementations such as T-HEAD's C906, C908, C910 and C920
supports efficient unaligned access, for performance reason we want
to enable HAVE_EFFICIENT_UNALIGNED_ACCESS on these platforms. To
avoid performance regressions on other non efficient unaligned access
platforms, HAVE_EFFICIENT_UNALIGNED_ACCESS can't be globaly selected.

To solve this problem, runtime code patching based on the detected
speed is a good solution. But that's not easy, it involves lots of
work to modify vairous subsystems such as net, mm, lib and so on.
This can be done step by step.

patch1 introduces RISCV_EFFICIENT_UNALIGNED_ACCESS which depends on
NONPORTABLE, if users know during config time that the kernel will be
only run on those efficient unaligned access hw platforms, they can
enable it. Obviously, generic unified kernel Image should enable it.

patch2 adds support DCACHE_WORD_ACCESS when MMU and
RISCV_EFFICIENT_UNALIGNED_ACCESS.

Below test program and step shows how much performance can be improved:

 $ cat tt.c
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>

 #define ITERATIONS 1000000

 #define PATH "123456781234567812345678123456781"

 int main(void)
 {
         unsigned long i;
         struct stat buf;

         for (i = 0; i < ITERATIONS; i++)
                 stat(PATH, &buf);

         return 0;
 }

 $ gcc -O2 tt.c
 $ touch 123456781234567812345678123456781
 $ time ./a.out

Per my test on T-HEAD C910 platforms, the above test performance is
improved by about 7.5%.

Jisheng Zhang (2):
  riscv: introduce RISCV_EFFICIENT_UNALIGNED_ACCESS
  riscv: select DCACHE_WORD_ACCESS for efficient unaligned access HW

 arch/riscv/Kconfig                      | 13 +++++++++++
 arch/riscv/include/asm/asm-extable.h    | 15 ++++++++++++
 arch/riscv/include/asm/word-at-a-time.h | 23 ++++++++++++++++++
 arch/riscv/mm/extable.c                 | 31 +++++++++++++++++++++++++
 4 files changed, 82 insertions(+)

-- 
2.42.0