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Message-ID: <20251009-secluded-writing-adefb1f71a72@spud>
Date: Thu, 9 Oct 2025 17:53:14 +0100
From: Conor Dooley <conor@...nel.org>
To: Yunhui Cui <cuiyunhui@...edance.com>
Cc: paul.walmsley@...ive.com, palmer@...belt.com, aou@...s.berkeley.edu,
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Subject: Re: [PATCH RFC] riscv: add support for Ziccid
On Thu, Oct 09, 2025 at 09:45:14PM +0800, Yunhui Cui wrote:
> The Ziccid extension provides hardware synchronization between
> Dcache and Icache. With this hardware support, there's no longer
> a need to trigger remote hart execution of fence.i via IPI.
>
> Signed-off-by: Yunhui Cui <cuiyunhui@...edance.com>
Actual correctness aside, in an RFC should you really state why this is
an RFC and not just a v1 patch. You're missing a dt-binding change
that's required for new extensions, that you'll need for v2.
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 67b59699357da..2da82aa2dbf0a 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -540,6 +540,7 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = {
> __RISCV_ISA_EXT_DATA(svnapot, RISCV_ISA_EXT_SVNAPOT),
> __RISCV_ISA_EXT_DATA(svpbmt, RISCV_ISA_EXT_SVPBMT),
> __RISCV_ISA_EXT_DATA(svvptc, RISCV_ISA_EXT_SVVPTC),
> + __RISCV_ISA_EXT_DATA(ziccid, RISCV_ISA_EXT_ZICCID),
> };
The comment about this structure reads:
/*
* The canonical order of ISA extension names in the ISA string is defined in
* chapter 27 of the unprivileged specification.
*
* Ordinarily, for in-kernel data structures, this order is unimportant but
* isa_ext_arr defines the order of the ISA string in /proc/cpuinfo.
*
* The specification uses vague wording, such as should, when it comes to
* ordering, so for our purposes the following rules apply:
*
* 1. All multi-letter extensions must be separated from other extensions by an
* underscore.
*
* 2. Additional standard extensions (starting with 'Z') must be sorted after
* single-letter extensions and before any higher-privileged extensions.
*
* 3. The first letter following the 'Z' conventionally indicates the most
* closely related alphabetical extension category, IMAFDQLCBKJTPVH.
* If multiple 'Z' extensions are named, they must be ordered first by
* category, then alphabetically within a category.
*
* 3. Standard supervisor-level extensions (starting with 'S') must be listed
* after standard unprivileged extensions. If multiple supervisor-level
* extensions are listed, they must be ordered alphabetically.
*
* 4. Standard machine-level extensions (starting with 'Zxm') must be listed
* after any lower-privileged, standard extensions. If multiple
* machine-level extensions are listed, they must be ordered
* alphabetically.
*
* 5. Non-standard extensions (starting with 'X') must be listed after all
* standard extensions. If multiple non-standard extensions are listed, they
* must be ordered alphabetically.
*
* An example string following the order is:
* rv64imadc_zifoo_zigoo_zafoo_sbar_scar_zxmbaz_xqux_xrux
*
* New entries to this struct should follow the ordering rules described above.
*/
Cheers,
Conor.
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