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: <877cz0nbce.fsf@oldenburg.str.redhat.com>
Date:   Fri, 09 Dec 2022 11:02:57 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Andrew Waterman <andrew@...ive.com>
Cc:     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, Darius Rad <darius@...espec.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)

* 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.

Thanks,
Florian