linux-kernel - Re: [PATCH v3 2/3] RISC-V: uapi: add HWCAP for Bitmanip/Scalar Crypto

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Date:   Thu, 24 Nov 2022 10:30:21 +0100
From:   Samuel Ortiz <sameo@...osinc.com>
To:     "Hongren (Zenithal) Zheng" <i@...ithal.me>
Cc:     Palmer Dabbelt <palmer@...osinc.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Atish Patra <atishp@...osinc.com>,
        Anup Patel <anup@...infault.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org,
        Michael Kerrisk <mtk.manpages@...il.com>,
        linux-man@...r.kernel.org, Jiatai He <jiatai2021@...as.ac.cn>,
        Heiko Stuebner <heiko@...ech.de>, Conor.Dooley@...rochip.com
Subject: Re: [PATCH v3 2/3] RISC-V: uapi: add HWCAP for Bitmanip/Scalar Crypto

On Mon, Jun 13, 2022 at 02:46:35AM +0800, Hongren (Zenithal) Zheng wrote:
> diff --git a/arch/riscv/include/uapi/asm/hwcap.h b/arch/riscv/include/uapi/asm/hwcap.h
> index 46dc3f5ee99f..bfed3e5c338c 100644
> --- a/arch/riscv/include/uapi/asm/hwcap.h
> +++ b/arch/riscv/include/uapi/asm/hwcap.h
> @@ -22,4 +22,26 @@
>  #define COMPAT_HWCAP_ISA_D	(1 << ('D' - 'A'))
>  #define COMPAT_HWCAP_ISA_C	(1 << ('C' - 'A'))
>  
> +/*
> + * HWCAP2 flags - for elf_hwcap2 (in kernel) and AT_HWCAP2
> + *
> + * As only 32 bits of elf_hwcap (in kernel) could be used
> + * and RISC-V has reserved 26 bits of it, other caps like
> + * bitmanip and crypto can not be placed in AT_HWCAP
> + */

Have we agreed that multi-letter ISA extensions would use hwcap to be
exposed to userspace? With so many potential extensions, we could
quickly run out of space on AT_HWCAP2 as well.

Cheers,
Samuel.