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

[Florian Weimer]

* 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).
It's possible to do this differently, but every mid-level startup code
will have to replicate it (the libcs, other run-time environments like
Go, and so on).

Still it's much better than executing the instruction to see if it
traps, so I won't complain too much.

Thanks,
Florian