[PATCH v4 0/1] riscv: improving uaccess with logs from network bench

[Akira Tsukamoto]

Hi Palmer,

Please do not bather with this patch.
It still have bug for rv32 reported by Guenter and I will regenerate the 
patch against v5.14-rc* since I made the patch against v5.13.x for this
patch.

Akira

On 7/19/2021 9:51 PM, Akira Tsukamoto wrote:
> Hi Guenter, Geert and Qiu,
> 
> I fixed the bug which was overrunning the copy when the size was in the
> between 8*SZREG to 9*SZREG. The SZREG holds the bytes per register size
> which is 4 for RV32 and 8 for RV64.
> 
> Do you mind trying this patch? It works OK at my place.
> 
> Since I had to respin the patch I added word copy without unrolling when
> the size is in the between 2*SZREG to 9*SZREG to reduce the number of byte
> copies which has heavy overhead as Palmer has mentioned when he included
> this patch to riscv/for-next.
> 
> 
> I rewrote the functions but heavily influenced by Garry's memcpy
> function [1]. It must be written in assembler to handle page faults
> manually inside the function unlike other memcpy functions.
> 
> This patch will reduce cpu usage dramatically in kernel space especially
> for applications which use sys-call with large buffer size, such as network
> applications. The main reason behind this is that every unaligned memory
> access will raise exceptions and switch between s-mode and m-mode causing
> large overhead.
> 
> ---
> v3 -> v4:
> - Fixed overrun copy
> - Added word copy without unrolling to reduce byte copy for left over
> 
> v2 -> v3:
> - Merged all patches
> 
> v1 -> v2:
> - Added shift copy
> - Separated patches for readability of changes in assembler
> - Using perf results
> 
> [1] https://lkml.org/lkml/2021/2/16/778
> 
> Akira Tsukamoto (1):
>   riscv: __asm_copy_to-from_user: Optimize unaligned memory access and
>     pipeline stall
> 
>  arch/riscv/lib/uaccess.S | 218 ++++++++++++++++++++++++++++++++-------
>  1 file changed, 183 insertions(+), 35 deletions(-)
>