linux-kernel - Re: [PATCH v5 0/6] Provide a fraemework for RISC-V ISA extensions

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Date:   Thu, 10 Mar 2022 15:50:55 -0800 (PST)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     Atish Patra <atishp@...osinc.com>
CC:     linux-kernel@...r.kernel.org, Atish Patra <atishp@...osinc.com>,
        aou@...s.berkeley.edu, atishp@...shpatra.org, anup@...infault.org,
        damien.lemoal@....com, devicetree@...r.kernel.org,
        jszhang@...nel.org, krzysztof.kozlowski@...onical.com,
        linux-riscv@...ts.infradead.org,
        Paul Walmsley <paul.walmsley@...ive.com>, robh+dt@...nel.org
Subject:     Re: [PATCH v5 0/6] Provide a fraemework for RISC-V ISA extensions 

On Tue, 22 Feb 2022 12:48:05 PST (-0800), Atish Patra wrote:
> This series implements a generic framework to parse multi-letter ISA
> extensions. This series is based on Tsukasa's v3 isa extension improvement
> series[1]. I have fixed few bugs and improved comments from that series
> (PATCH1-3). I have not used PATCH 4 from that series as we are not using
> ISA extension versioning as of now. We can add that later if required.
>
> PATCH 4 allows the probing of multi-letter extensions via a macro.
> It continues to use the common isa extensions between all the harts.
> Thus hetergenous hart systems will only see the common ISA extensions.
>
> PATCH 6 improves the /proc/cpuinfo interface for the available ISA extensions
> via /proc/cpuinfo.
>
> Here is the example output of /proc/cpuinfo:
> (with debug patches in Qemu and Linux kernel)
>
> # cat /proc/cpuinfo
> processor	: 0
> hart		: 0
> isa		: rv64imafdch
> isa-ext		: svpbmt svnapot svinval

I know it might seem a bit pedantic, but I really don't want to 
introduce a new format for encoding ISA extensions -- doubly so if this 
is the only way we're giving this info to userspace, as then we're just 
asking folks to turn this into a defacto ABI.  Every time we try to do 
something that's sort of like an ISA string but not exactly what's in 
the spec we end up getting burned, and while I don't see a specific way 
that could happen here that's what's happened so many times before I 
just don't want to risk it.

I've gone ahead and removed some of this information (isa-ext, and all 
the single-letter extensions we can't properly turn on yet) from 
/proc/cpuinfo, modifying the last patch to do so.  It's at 
palmer/riscv-isa, LMK if that's OK.

I'm not opposed to doing something here, I just really don't want to 
rush into it as we've already got enough complexity around the various 
flavors of ISA strings.  I don't see a big pressing need to provide this 
information to userspace, but having the rest of this sorted out is 
great (and there's some dependencies on this) so I'd prefer to just 
stick to what we know is good.

> mmu		: sv48
>
> processor	: 1
> hart		: 1
> isa		: rv64imafdch
> isa-ext		: svpbmt svnapot svinval
> mmu		: sv48
>
> processor	: 2
> hart		: 2
> isa		: rv64imafdch
> isa-ext		: svpbmt svnapot svinval
> mmu		: sv48
>
> processor	: 3
> hart		: 3
> isa		: rv64imafdch
> isa-ext		: svpbmt svnapot svinval
> mmu		: sv48
>
> Anybody adding support for any new multi-letter extensions should add an
> entry to the riscv_isa_ext_id and the isa extension array.
> E.g. The patch[2] adds the support for various ISA extensions.
>
> [1] https://lore.kernel.org/all/0f568515-a05e-8204-aae3-035975af3ee8@irq.a4lg.com/T/
> [2] https://github.com/atishp04/linux/commit/e9e240c9a854dceb434ceb53bdbe82a657bee5f2
>
> Changes from v4->v5:
> 1. Improved the /proc/cpuinfo to include only valid & enabled extensions
> 2. Improved the multi-letter parsing by skipping the 'su' modes generated in
>    Qemu as suggested by Tsukasa.
>
> Changes from v3->v4:
> 1. Changed temporary variable for current hart isa to a bitmap
> 2. Added reviewed-by tags.
> 3. Improved comments
>
> Changes from v2->v3:
> 1. Updated comments to mark clearly a fix required for Qemu only.
> 2. Fixed a bug where the 1st multi-letter extension can be present without _
> 3. Added Tested by tags.
>
> Changes from v1->v2:
> 1. Instead of adding a separate DT property use the riscv,isa property.
> 2. Based on Tsukasa's v3 isa extension improvement series.
>
> Atish Patra (3):
> RISC-V: Implement multi-letter ISA extension probing framework
> RISC-V: Do no continue isa string parsing without correct XLEN
> RISC-V: Improve /proc/cpuinfo output for ISA extensions
>
> Tsukasa OI (3):
> RISC-V: Correctly print supported extensions
> RISC-V: Minimal parser for "riscv, isa" strings
> RISC-V: Extract multi-letter extension names from "riscv, isa"
>
> arch/riscv/include/asm/hwcap.h |  25 +++++++
> arch/riscv/kernel/cpu.c        |  51 ++++++++++++-
> arch/riscv/kernel/cpufeature.c | 130 +++++++++++++++++++++++++++------
> 3 files changed, 183 insertions(+), 23 deletions(-)