linux-kernel - RISCV Vector unit disabled by default for new task (was Re: [PATCH v12 17/17] riscv: prctl to enable vector commands)

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Message-ID: <b1dae947-d52a-d28e-5ddc-c1ad6d29828c@rivosinc.com>
Date:   Thu, 8 Dec 2022 21:16:06 -0800
From:   Vineet Gupta <vineetg@...osinc.com>
To:     Chris Stillson <stillson@...osinc.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>,
        Guo Ren <guoren@...nel.org>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Greentime Hu <greentime.hu@...ive.com>,
        Vincent Chen <vincent.chen@...ive.com>,
        Andy Chiu <andy.chiu@...ive.com>,
        Andrew Waterman <andrew@...ive.com>,
        Darius Rad <darius@...espec.com>,
        Arnd Bergmann <arnd@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Björn Töpel <bjorn@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        GNU C Library <libc-alpha@...rceware.org>,
        Conor Dooley <Conor.Dooley@...rochip.com>,
        Christoph Müllner <christoph.muellner@...ll.eu>,
        Aaron Durbin <adurbin@...osinc.com>, linux@...osinc.com
Subject: RISCV Vector unit disabled by default for new task (was Re: [PATCH
 v12 17/17] riscv: prctl to enable vector commands)

Hi Darius, Andrew, Palmer

On 9/21/22 14:43, Chris Stillson wrote:
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
>
> @@ -134,7 +135,6 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
>   			if (WARN_ON(!vstate->datap))
>   				return;
>   		}
> -		regs->status |= SR_VS_INITIAL;
>   

Perhaps not obvious from the patch, but this is a major user experience 
change: As in V unit would be turned off for a new task and we will rely 
on a userspace prctl (also introduced in this patch) to enable V.

I know some of you had different opinion on this in the past [1], so 
this is to make sure everyone's on same page.
And if we agree this is the way to go, how exactly will this be done in 
userspace.

glibc dynamic loader will invoke the prctl() ? How will it decide 
whether to do this (or not) - will it be unconditional or will it use 
the hwcap - does latter plumbing exist already ? If so is it AT_HWCAP / 
HWCAP2.

Also for static linked executables, where will the prctl be called from ?

[1] https://sourceware.org/pipermail/libc-alpha/2021-November/132883.html