linux-kernel - Re: [PATCH v4 2/4] RISC-V: Add riscv_isa reprensenting ISA features common across CPUs

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Date:   Tue, 6 Aug 2019 14:54:49 -0700 (PDT)
From:   Paul Walmsley <paul.walmsley@...ive.com>
To:     Atish Patra <atish.patra@....com>, Anup Patel <anup.patel@....com>
cc:     linux-kernel@...r.kernel.org, Alan Kao <alankao@...estech.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        devicetree@...r.kernel.org, Enrico Weigelt <info@...ux.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johan Hovold <johan@...nel.org>,
        linux-riscv@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Rob Herring <robh+dt@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 2/4] RISC-V: Add riscv_isa reprensenting ISA features
 common across CPUs

Hi Anup, Atish,

On Fri, 2 Aug 2019, Atish Patra wrote:

> From: Anup Patel <anup.patel@....com>
> 
> This patch adds riscv_isa integer to represent ISA features common
> across all CPUs. The riscv_isa is not same as elf_hwcap because
> elf_hwcap will only have ISA features relevant for user-space apps
> whereas riscv_isa will have ISA features relevant to both kernel
> and user-space apps.
> 
> One of the use case is KVM hypervisor where riscv_isa will be used
> to do following operations:
> 
> 1. Check whether hypervisor extension is available
> 2. Find ISA features that need to be virtualized (e.g. floating
>    point support, vector extension, etc.)
> 
> Signed-off-by: Anup Patel <anup.patel@....com>
> Signed-off-by: Atish Patra <atish.patra@....com>

Do you have any opinions on how this patch might change for the Z-prefix 
extensions?  This bitfield approach probably won't scale, and with the 
EXPORT_SYMBOL(), it might be worth trying to put together a approach that 
would work over the long term?


- Paul