linux-kernel - Re: [PATCH v3 0/6] Provide a fraemework for RISC-V ISA extensions

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Date:   Tue, 15 Feb 2022 23:56:57 +0800
From:   Jisheng Zhang <jszhang@...nel.org>
To:     Atish Patra <atishp@...osinc.com>
Cc:     linux-kernel@...r.kernel.org, Albert Ou <aou@...s.berkeley.edu>,
        Atish Patra <atishp@...shpatra.org>,
        Anup Patel <anup@...infault.org>,
        Damien Le Moal <damien.lemoal@....com>,
        devicetree@...r.kernel.org,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        linux-riscv@...ts.infradead.org,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v3 0/6] Provide a fraemework for RISC-V ISA extensions

On Tue, Feb 15, 2022 at 01:02:05AM -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		: rv64imafdcsu
> isa-ext		: sstc,sscofpmf
> mmu		: sv48
> 
> processor	: 1
> hart		: 1
> isa		: rv64imafdcsu
> isa-ext		: sstc,sscofpmf
> mmu		: sv48
> 
> processor	: 2
> hart		: 2
> isa		: rv64imafdcsu
> isa-ext		: sstc,sscofpmf
> mmu		: sv48
> 
> processor	: 3
> hart		: 3
> isa		: rv64imafdcsu
> isa-ext		: sstc,sscofpmf
> 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.

Hi Atish,

Thanks for this series. I'm thinking cpu features VS ISA extenstions.
I'm converting the sv48 to static key:
https://lore.kernel.org/linux-riscv/20220125165036.987-1-jszhang@kernel.org/

Previously, I thought the SV48 as a cpu feature, and there will be
more and more cpu features, so I implemented an unified static key
mechanism for CPU features. But after reading this series, I think
I may need to rebase(even reimplement) the above patch to your series.
But I'm a bit confused by CPU features VS ISA extenstions now:

1. Is cpu feature  == ISA extension?

2. Is SV48 considered as ISA extension?
If yes, now SV48 or not is determined during runtime, but current ISA
extensions seem parsed from DT. So how to support those ISA extensions
which can be determined during runtime?

Could you please share your thought?

Thanks