[PATCH 0/4] Zbb + fast-unaligned string optimization

[Palmer Dabbelt]

On Fri, 13 Jan 2023 13:23:47 PST (-0800), heiko at sntech.de wrote:
> From: Heiko Stuebner <heiko.stuebner at vrull.eu>
>
> This is a follow-up to my Zbb-based string optimization series, that
> then adds another strcmp variant for systems with Zbb that also can
> do unaligned accesses fast in hardware.
>
> For this it uses Palmer's series for hw-feature probing that would read
> this property from firmware (devicetree), as the performance of unaligned
> accesses is an implementation detail of the relevant cpu core.
>
>
> Right now we're still in the middle of discussing how more complex
> cpufeature-combinations should be handled in general, so this is more
> of a concept on one possible way to do it.

Sorry for leaving this dormant for a bit.  There's been a lot of 
discussions and I think the general consensus is to aim at taking these 
combined workloads only if they are a performance win on real hardware.

I think there's no Zbb+fast-unaligned hardware availiable today, but I'm 
not 100% sure on that.  If there is and someone can show benchmarks then 
I'm happy to fit something like this in somehow, but otherwise I think 
we should wait and see if this matches what ships.

> Dependencies:
> - my Zbb string series
>   https://lore.kernel.org/r/20230113212301.3534711-1-heiko@sntech.de
> - Palmer's hw-probing series
>   https://lore.kernel.org/r/20221013163551.6775-1-palmer@rivosinc.com
>
>
> Heiko Stuebner (4):
>   RISC-V: use bit-values instead of numbers to identify patched
>     cpu-features
>   RISC-V: add alternative-field for bits to not match against
>   RISC-V: add cpufeature probing for fast-unaligned access
>   RISC-V: add strcmp variant using zbb and fast-unaligned access
>
>  arch/riscv/include/asm/alternative-macros.h |  64 ++++----
>  arch/riscv/include/asm/alternative.h        |   1 +
>  arch/riscv/include/asm/errata_list.h        |  27 ++--
>  arch/riscv/kernel/cpufeature.c              |  33 +++-
>  arch/riscv/lib/strcmp.S                     | 170 +++++++++++++++++++-
>  arch/riscv/lib/strlen.S                     |   2 +-
>  arch/riscv/lib/strncmp.S                    |   2 +-
>  7 files changed, 245 insertions(+), 54 deletions(-)