linux-kernel - Re: 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: <Y5MoPdYimQtqQvkM@bruce.bluespec.com>
Date:   Fri, 9 Dec 2022 07:21:17 -0500
From:   Darius Rad <darius@...espec.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Andrew Waterman <andrew@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Vineet Gupta <vineetg@...osinc.com>, stillson@...osinc.com,
        Paul Walmsley <paul.walmsley@...ive.com>, anup@...infault.org,
        atishp@...shpatra.org, guoren@...nel.org,
        Conor Dooley <conor.dooley@...rochip.com>,
        greentime.hu@...ive.com, vincent.chen@...ive.com,
        andy.chiu@...ive.com, arnd@...nel.org,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        bjorn@...nel.org, libc-alpha@...rceware.org,
        christoph.muellner@...ll.eu, Aaron Durbin <adurbin@...osinc.com>,
        linux@...osinc.com
Subject: Re: RISCV Vector unit disabled by default for new task (was Re:
 [PATCH v12 17/17] riscv: prctl to enable vector commands)

On Fri, Dec 09, 2022 at 11:02:57AM +0100, Florian Weimer wrote:
> * Andrew Waterman:
> 
> > This suggests that ld.so, early-stage libc, or possibly both will need
> > to make this prctl() call, perhaps by parsing the ELF headers of the
> > binary and each library to determine if the V extension is used.
> 
> If the string functions use the V extension, it will be enabled
> unconditionally.  So I don't see why it's okay for libc to trigger this
> alleged UAPI change, when the kernel can't do it by default.
> 

Because the call to enable can fail and userspace needs to deal with that.

// darius