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Date:   Thu, 24 Nov 2022 11:47:30 +0100
From:   Samuel Ortiz <sameo@rivosinc.com>
To:     Conor Dooley <conor.dooley@microchip.com>
Cc:     "Hongren (Zenithal) Zheng" <i@zenithal.me>,
        Palmer Dabbelt <palmer@rivosinc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Atish Patra <atishp@rivosinc.com>,
        Anup Patel <anup@brainfault.org>,
        Eric Biederman <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>, linux-mm@kvack.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        linux-man@vger.kernel.org, Jiatai He <jiatai2021@iscas.ac.cn>,
        Heiko Stuebner <heiko@sntech.de>
Subject: Re: [PATCH v3 2/3] RISC-V: uapi: add HWCAP for Bitmanip/Scalar Crypto
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On Thu, Nov 24, 2022 at 09:58:53AM +0000, Conor Dooley wrote:
> On Thu, Nov 24, 2022 at 10:30:21AM +0100, Samuel Ortiz wrote:
> > 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.
> 
> Palmer whipped up a PoC hwprobe interface (during Plumbers I think) that
> Heiko is currently looking into - I think his motivation is misaligned
> access performance. There's a branch but I have no idea if it even
> compiles... I'm mostly waiting for whatever Heiko comes up with ;)
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/palmer/linux.git/log/?h=riscv-hwprobe-v1
> 
> This patchset seems to need a rebase anyway per your other reply, but I
> guess that the new proposed interface would be preferable?
I think so, yes.
Patch #1 is definitely needed regardless of which interface we pick for
exposing the ISA strings to userspace.

Cheers,
Samuel.