RFR: 8309258: RISC-V: Add riscv_hwprobe syscall [v6]

[Fei Yang]

On Thu, 15 Jun 2023 16:30:00 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/
>
> Robbin Ehn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   rivos mvendorid was not proper encoded.

Changes requested by fyang (Reviewer).

src/hotspot/os_cpu/linux_riscv/vm_version_linux_riscv.cpp line 196:

> 194:         if (strncmp(buf, "uarch", sizeof "uarch" - 1) == 0) {
> 195:           ret = os::strdup(p + 2);
> 196:           ret[strcspn(p, "\n")] = '\0';

This line was triggering some NMT corruption errors when running tier1 test on unmatched board. I think this should be: `ret[strcspn(ret, "\n")] = '\0';`.

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PR Review: https://git.openjdk.org/jdk/pull/14445#pullrequestreview-1485544356
PR Review Comment: https://git.openjdk.org/jdk/pull/14445#discussion_r1233616135