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: <Y5MtTLtrDnCW6Dik@bruce.bluespec.com>
Date:   Fri, 9 Dec 2022 07:42:52 -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 01:32:33PM +0100, Florian Weimer via Libc-alpha wrote:
> * Darius Rad:
> 
> > 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.
> 
> Failure is usually indicated by an AT_HWCAP or AT_HWCAP2 bit remaining
> zero, or perhaps a special CPU register (although that is more unusual).

That would indicate that the extension is not present, which is one of, but
not the only way it can fail.

The vector extension relies on dynamically allocated memory in the kernel,
which can fail.

It also provides the opportunity for the kernel to deny access to the
vector extension, perhaps due to administrative policy or other future
mechanism.

// darius