linux-kernel - Re: [PATCH v2 0/4] unified way to use static key and optimize pgtable_l4

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Date:   Mon, 9 May 2022 10:07:16 +0530
From:   Anup Patel <anup@...infault.org>
To:     Jisheng Zhang <jszhang@...nel.org>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Alexandre Ghiti <alexandre.ghiti@...onical.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        kasan-dev@...glegroups.com
Subject: Re: [PATCH v2 0/4] unified way to use static key and optimize pgtable_l4_enabled

On Sun, May 8, 2022 at 9:46 PM Jisheng Zhang <jszhang@...nel.org> wrote:
>
> Currently, riscv has several features which may not be supported on all
> riscv platforms, for example, FPU, SV48, SV57 and so on. To support
> unified kernel Image style, we need to check whether the feature is
> suportted or not. If the check sits at hot code path, then performance
> will be impacted a lot. static key can be used to solve the issue. In
> the past, FPU support has been converted to use static key mechanism.
> I believe we will have similar cases in the future. For example, the
> SV48 support can take advantage of static key[1].
>
> patch1 is a simple W=1 warning fix.
> patch2 introduces an unified mechanism to use static key for riscv cpu
> features.
> patch3 converts has_cpu() to use the mechanism.
> patch4 uses the mechanism to optimize pgtable_l4|[l5]_enabled.
>
> [1] http://lists.infradead.org/pipermail/linux-riscv/2021-December/011164.html

Overall, using a script to generate CPU capabilities seems a bit
over-engineered to me. We already have RISC-V ISA extension
parsing infrastructure which can be easily extended to support
static key arrays.

Regards,
Anup

>
> Since v1:
>  - Add a W=1 warning fix
>  - Fix W=1 error
>  - Based on v5.18-rcN, since SV57 support is added, so convert
>    pgtable_l5_enabled as well.
>
> Jisheng Zhang (4):
>   riscv: mm: init: make pt_ops_set_[early|late|fixmap] static
>   riscv: introduce unified static key mechanism for CPU features
>   riscv: replace has_fpu() with system_supports_fpu()
>   riscv: convert pgtable_l4|[l5]_enabled to static key
>
>  arch/riscv/Makefile                 |   3 +
>  arch/riscv/include/asm/cpufeature.h | 110 ++++++++++++++++++++++++++++
>  arch/riscv/include/asm/pgalloc.h    |  16 ++--
>  arch/riscv/include/asm/pgtable-64.h |  40 +++++-----
>  arch/riscv/include/asm/pgtable.h    |   5 +-
>  arch/riscv/include/asm/switch_to.h  |   9 +--
>  arch/riscv/kernel/cpu.c             |   4 +-
>  arch/riscv/kernel/cpufeature.c      |  29 ++++++--
>  arch/riscv/kernel/process.c         |   2 +-
>  arch/riscv/kernel/signal.c          |   4 +-
>  arch/riscv/mm/init.c                |  52 ++++++-------
>  arch/riscv/mm/kasan_init.c          |  16 ++--
>  arch/riscv/tools/Makefile           |  22 ++++++
>  arch/riscv/tools/cpucaps            |   7 ++
>  arch/riscv/tools/gen-cpucaps.awk    |  40 ++++++++++
>  15 files changed, 274 insertions(+), 85 deletions(-)
>  create mode 100644 arch/riscv/include/asm/cpufeature.h
>  create mode 100644 arch/riscv/tools/Makefile
>  create mode 100644 arch/riscv/tools/cpucaps
>  create mode 100755 arch/riscv/tools/gen-cpucaps.awk
>
> --
> 2.34.1
>