RFR: 8347343: RISC-V: Unchecked zicntr csr reads

[Fei Yang]

On Thu, 9 Jan 2025 12:55:45 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

> Hi, please consider.
> 
> The rdinstret/rdcycle/rdtime are really useful and would be great to add in e.g. JFR.
> But as of now they are not used and zicntr is not yet in hwprobe therefore this patch just make sure they can't be used.
> 
> If we need them before hwprobe we can add a flag for manually enabling them.
> 
> Sanity tested.
> 
> Thanks!

Thanks. One minor comment.

src/hotspot/cpu/riscv/macroAssembler_riscv.cpp line 1338:

> 1336: 
> 1337: void MacroAssembler::csrr(Register Rd, unsigned csr) {
> 1338:   assert((csr != CSR_INSTRET && csr != CSR_CYCLE && csr != CSR_TIME) || VM_Version::ext_Zicntr.enabled(), "sanity");

Personally, I perfer to check a JVM option here (Say `UseZicntr` of diagnostic type). When we have hwprobe for this extension, we can further hook up this option to `VM_Version::ext_Zicntr`. And the user could still enable on command line before that. What do you think?

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PR Review: https://git.openjdk.org/jdk/pull/23003#pullrequestreview-2541469983
PR Review Comment: https://git.openjdk.org/jdk/pull/23003#discussion_r1909743262