[v4,0/5] riscv: uaccess: optimizations

Message

Cyril Bur March 18, 2025, 6:15 a.m. UTC

This series tries to optimize riscv uaccess by allowing the use of
user_access_begin() and user_access_end() which permits grouping user accesses
and avoiding the CSR write penalty for each access.

The error path can also be optimised using asm goto which patches 3 and 4
achieve. This will speed up jumping to labels by avoiding the need of an
intermediary error type variable within the uaccess macros

I did read the discussion this series generated. It isn't clear to me
which direction to take the patches, if any.

V2:
I've taken on this series as there isn't any response from Jisheng. No
significant changes other than build fixes.
- Fixes build breakage in patch 3 to do with not having used 'goto' keyword.
- Fixes build breakage in patch 4 on 32bit not having delcared __ptr in the
  macro.

V3:
Significant commit message rewrites.
 - Corrected the justification for patch 2
 - Better explained/justified patches 3 and 4
Minor code changes for legibility and more comments

V4:
Fixed checkpatch errors
Added a unsafe_copy_from_user()
Added patch from Ben Dooks to save SR_SUM bit on switch

Ben Dooks (1):
  riscv: save the SR_SUM status over switches

Jisheng Zhang (4):
  riscv: implement user_access_begin() and families
  riscv: uaccess: use input constraints for ptr of __put_user()
  riscv: uaccess: use 'asm goto' for put_user()
  riscv: uaccess: use 'asm_goto_output' for get_user()

 arch/riscv/include/asm/processor.h |   1 +
 arch/riscv/include/asm/uaccess.h   | 218 ++++++++++++++++++++++-------
 arch/riscv/kernel/asm-offsets.c    |   5 +
 arch/riscv/kernel/entry.S          |   8 ++
 4 files changed, 179 insertions(+), 53 deletions(-)

Comments

Cyril Bur March 19, 2025, 1:56 a.m. UTC | #1

On 18/3/2025 5:15 pm, Cyril Bur wrote:
> This series tries to optimize riscv uaccess by allowing the use of
> user_access_begin() and user_access_end() which permits grouping user accesses
> and avoiding the CSR write penalty for each access.
> 
> The error path can also be optimised using asm goto which patches 3 and 4
> achieve. This will speed up jumping to labels by avoiding the need of an
> intermediary error type variable within the uaccess macros
> 
> I did read the discussion this series generated. It isn't clear to me
> which direction to take the patches, if any.
> 
> V2:
> I've taken on this series as there isn't any response from Jisheng. No
> significant changes other than build fixes.
> - Fixes build breakage in patch 3 to do with not having used 'goto' keyword.
> - Fixes build breakage in patch 4 on 32bit not having delcared __ptr in the
>    macro.
> 
> V3:
> Significant commit message rewrites.
>   - Corrected the justification for patch 2
>   - Better explained/justified patches 3 and 4
> Minor code changes for legibility and more comments
> 
> V4:
> Fixed checkpatch errors
> Added a unsafe_copy_from_user()
> Added patch from Ben Dooks to save SR_SUM bit on switch

There is something uniquely wrong with V4 (not sure of the cause yet). I 
will send a new version.

> 
> Ben Dooks (1):
>    riscv: save the SR_SUM status over switches
> 
> Jisheng Zhang (4):
>    riscv: implement user_access_begin() and families
>    riscv: uaccess: use input constraints for ptr of __put_user()
>    riscv: uaccess: use 'asm goto' for put_user()
>    riscv: uaccess: use 'asm_goto_output' for get_user()
> 
>   arch/riscv/include/asm/processor.h |   1 +
>   arch/riscv/include/asm/uaccess.h   | 218 ++++++++++++++++++++++-------
>   arch/riscv/kernel/asm-offsets.c    |   5 +
>   arch/riscv/kernel/entry.S          |   8 ++
>   4 files changed, 179 insertions(+), 53 deletions(-)
>