RFR: 8309258: RISC-V: Add riscv_hwprobe syscall

[Robbin Ehn]

On Tue, 13 Jun 2023 09:58:24 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

> Hi, please consider.
> 
> Linux kernel 6.4 will come with the new syscall https://docs.kernel.org/riscv/hwprobe.html to determine CPU and features.
> RV cpus features/capabilities can vastly differ and it is not feasible for users to manually enable the correct feature set.
> Today the VM uses the ELF aux vector and cpuinfo to gather some information about CPU capabilities.
> 
> Currently features are track with a bit field, this is insufficient.
> There are many capabilities and these can have values attached to them.
> CPU features should also be possible to turn if we can determine vendor (hwprobe).
> 
> This patchs adds the syscall, uses the syscall in combination with the aux and cpuinfo to enable features by default.
> If there is a vendor specific path it calls that in addition.
> Then we build the feature string(and bit field) and update flags accordingly.
> 
> Tested t1 and hwprobe with:
> https://lore.kernel.org/qemu-devel/7f8d733df6e9b6151e9efb843d55441348805e70.camel@rivosinc.com/

Thanks @markdryan for pointing out that we should free the uarch string.

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PR Comment: https://git.openjdk.org/jdk/pull/14445#issuecomment-1589210962