RFR: 8333245: RISC-V: UseRVV option can't be enabled after JDK-8316859

[Robbin Ehn]

On Thu, 27 Jun 2024 10:04:30 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

> We need to update our check from hwcap with something along the line of:
> 
> ```
>   vsetvli t0, zero, e8, m1, ta, ma
>   csrr a0, vtype
>   sgtz a0, a0
>   ret
> ```
> 
> That will return whether v1.0 is supported since `vsetvli t0, zero, e8, m1, ta, ma` will set the `vill` flag on RVV 0.7.

As we can be context switched after vsetvli and thus execute csrr on another hart, without kernel support, the flag will not be set even if it's RVV 0.7. So this check is not reliable.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/19472#issuecomment-2194324396