linux-kernel - Re: [PATCH v3 0/5] riscv: Use __riscv_has_extension

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Message-ID: <20251011-buffing-never-4911edc10600@spud>
Date: Sat, 11 Oct 2025 12:28:34 +0100
From: Conor Dooley <conor@...nel.org>
To: Vivian Wang <wangruikang@...as.ac.cn>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
	Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Paul Walmsley <pjw@...nel.org>,
	Charlie Jenkins <charlie@...osinc.com>,
	Xiao Wang <xiao.w.wang@...el.com>,
	Christoph Müllner <christoph.muellner@...ll.eu>,
	Vivian Wang <uwu@...m.page>, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/5] riscv: Use __riscv_has_extension_{likely,unlikely}

On Sat, Oct 11, 2025 at 07:35:42AM +0800, Vivian Wang wrote:
> There are about a dozen uses of asm goto in arch/riscv just to select
> between two code paths with the alternative mechanism. Convert them to
> the existing helpers __riscv_has_extension_{likely,unlikely}.
> 
> In each case, I have preserved the existing code's choice of asm goto
> pattern while picking between "likely" and "unlikely", namely:
> 
>   ALTERNATIVE("j %l[no]", "nop", ...)   -> "likely"
>   ALTERNATIVE("nop", "j %l[yes]", ...)  -> "unlikely"
> 
> Since the helpers are just implementations of these patterns, the
> performance should be the same as before.

Can you explain why you're opting for the __ variant, instead of the one
without __? They should do the same thing in your cases, and resolve to
the alternative, since the non-alternative function call will be
unreachable and the assert is compiletime. There's currently no users of
the __ prefixed versions outside of other extension detection helpers, and
I think it should probably be kept that way if possible.

> 
> These patches are also available at:
> 
> https://github.com/dramforever/linux/tree/riscv/altn-helper/v2
> 
> ---
> Changes in v3:
> - Rebased on riscv for-next
>   - Resolve conflict, use ALT_RISCV_PAUSE() in moved lines
> - Link to v2: https://lore.kernel.org/r/20250821-riscv-altn-helper-wip-v2-0-9586fa702f78@iscas.ac.cn
> 
> Changes in v2:
> - Cc'd authors who initially introduced the asm goto blocks
> - Use existing __riscv_has_extension_{likely,unlikely} instead
> - Remove bogus comment for Zbb being likely (checksum)
> - Restructured patch to minimize diff (bitops, hweight, cmpxchg)
> - Link to v1: https://lore.kernel.org/r/20250820-riscv-altn-helper-wip-v1-0-c3c626c1f7e6@iscas.ac.cn
> 
> ---
> Vivian Wang (5):
>       riscv: pgtable: Use __riscv_has_extension_unlikely
>       riscv: checksum: Use __riscv_has_extension_likely
>       riscv: hweight: Use __riscv_has_extension_likely
>       riscv: bitops: Use __riscv_has_extension_likely
>       riscv: cmpxchg: Use __riscv_has_extension_likely
> 
>  arch/riscv/include/asm/arch_hweight.h | 24 ++++++----------
>  arch/riscv/include/asm/bitops.h       | 32 ++++++---------------
>  arch/riscv/include/asm/checksum.h     | 13 +++------
>  arch/riscv/include/asm/cmpxchg.h      | 12 +++-----
>  arch/riscv/include/asm/pgtable.h      | 15 +++++-----
>  arch/riscv/lib/csum.c                 | 53 ++++++++---------------------------
>  arch/riscv/mm/pgtable.c               | 22 +++++++--------
>  7 files changed, 53 insertions(+), 118 deletions(-)
> ---
> base-commit: cd5a0afbdf8033dc83786315d63f8b325bdba2fd
> change-id: 20250820-riscv-altn-helper-wip-00af3a552c37
> 
> Best regards,
> -- 
> Vivian "dramforever" Wang
> 

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