RFR: 8316859: RISC-V: Disable detection of V through HWCAP

[Robbin Ehn]

On Mon, 25 Sep 2023 08:38:03 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

> Some dev boards only support RVV version 0.7, while the OpenJDK only supports RVV version 1.0. These two versions are not compatible with each other. Given the V bit is set through HWCAP on some custom kernels, regardless of the version, it can lead to generating V instructions on boards that don't support RVV version 1.0 (ex: Sipeed LicheePi), leading to a SIGILL.
> 
> That is an acceptable workaround as only Linux Kernel v6.5+ supports V, and that version already support hwprobe anyway

V should be disabled in vcsr pre-6.5. Even if you manage somehow to enabled it you still need all registers saved and restored on a context switch, e.i. kernel must be aware of a user space program using V.

As you say, an aware kernel also supports hw probe, sensible, thanks!

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Marked as reviewed by rehn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/15898#pullrequestreview-1641647058