Re: [PATCH 0/2] riscv: enable EFFICIENT_UNALIGNED_ACCESS and DCACHE_WORD_ACCESS

Jisheng Zhang
2023-12-02T11:54:53Z

On Sat, Dec 02, 2023 at 07:18:20PM +0800, Jisheng Zhang wrote:
> 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.

Adding something as below here can make the series more clear:
So let's take an easier solution: add support to efficient unaligned
access and hide the support under NONPORTABLE.

>
> 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.

typo: s/should/shouldn't

>
> 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
>
>
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