[riscv-port] RFR: 8277968: riscv: Detect vector extension with vcsr [v2]

[Fei Yang]

On Tue, 30 Nov 2021 08:04:52 GMT, kuaiwei <duke at openjdk.java.net> wrote:

>> UseRVV could cause crash on D1 board(RISCV-C906).
>> 
>> It seems that though the D1 board is equipped with RVV-0.7.1[1] . In our test, VLENB CSR can return value of 16 on D1 board. So JDK will assume it can support RVV extension and crash in vector instructions when  UseRVV is enabled.  
>> 
>> RVV-0.9 and above[2] introduce a new VCSR CSR register, it will raise SIGILL on D1 board. So we can check it to detect vext support.
>> 
>> 
>> [1] https://github.com/riscv/riscv-v-spec/blob/0a24d0f61b5cd3f1f9265e8c40ab211daa865ede/v-spec.adoc#vector-extension-programmers-model
>> [2] https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#vector-extension-programmers-model
>
> kuaiwei has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR.

This might unnecessarily complicated the issue tbh. If you want to distinguish the specific cpu/core vendor, I would suggest we do in some different way. One possible way would be through checking /proc/cpuinfo for this infomation like aarch64 port [1]. I see boards like hifive unleashed and unmatched also provide necessary microarchitecure version through /proc/cpuinfo [2]. I think this kind of infomation should be made available in the system.

[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/os_cpu/linux_aarch64/vm_version_linux_aarch64.cpp#L146
[2] https://t2sde.org/hardware/board/SiFive/Unmatched

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PR: https://git.openjdk.java.net/riscv-port/pull/18