RFR: 8367137: RISC-V: Detect Zicboz block size via hwprobe [v2]

[Fei Yang]

On Tue, 9 Sep 2025 04:29:41 GMT, Dingli Zhang <dzhang at openjdk.org> wrote:

>> Hi,
>> Can you help to review this patch? Thanks!
>> 
>> We can extends RISC-V hwprobe support to include `RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE`.
>> The probed value is recorded in VM_Version::zicboz_block_size and then used to set the global CacheLineSize.
>> This ensures correct usage of the Zicboz extension, as block zeroing instructions operate on cache-line granularity.
>> 
>> FYI: https://docs.kernel.org/arch/riscv/hwprobe.html
>
> Dingli Zhang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix typo

src/hotspot/cpu/riscv/vm_version_riscv.cpp line 184:

> 182:   }
> 183: 
> 184:   if (UseZicboz) {

I don't think it's safe to turn on `UseBlockZeroing` without knowing the actual cache block size.
So we might want to add check for that here. Maybe change this if condition into something like:

if (UseZicboz && (UseZic64b || zicboz_block_size.enabled())) {

src/hotspot/cpu/riscv/vm_version_riscv.cpp line 187:

> 185:     if (zicboz_block_size.enabled()) {
> 186:       assert(UseZic64b ? zicboz_block_size.value() == 64 : true, "Zicboz block size should be 64 when UseZic64b is true");
> 187:       assert(is_power_of_2(zicboz_block_size.value()), "CacheLineSize must be a power of 2");

Can we merge the two assertions into one? Like:
`assert(UseZic64b ? zicboz_block_size.value() == 64 : is_power_of_2(zicboz_block_size.value()), "Sanity");`

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PR Review Comment: https://git.openjdk.org/jdk/pull/27155#discussion_r2332218887
PR Review Comment: https://git.openjdk.org/jdk/pull/27155#discussion_r2332213815