RFR: 8370947: Mitigate Neoverse-N1 erratum 1542419 negative impact on GCs and JIT performance [v21]

[Kim Barrett]

On Mon, 29 Dec 2025 21:51:20 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

>> Arm Neoverse N1 erratum 1542419: "The core might fetch a stale instruction from memory which violates the ordering of instruction fetches". It is fixed in Neoverse N1 r4p1.
>>  
>> Neoverse-N1 implementations mitigate erratum 1542419 with a workaround:
>> - Disable coherent icache.
>> - Trap IC IVAU instructions.
>> - Execute:
>>    - `tlbi vae3is, xzr`
>>    - `dsb sy`
>>  
>>  `tlbi vae3is, xzr` invalidates translations for all address spaces (global for address).  It waits for all memory accesses using in-scope old translation information to complete before it is considered complete.
>>  
>> As this workaround has significant overhead, Arm Neoverse N1 (MP050) Software Developer Errata Notice version 29.0 suggests:
>> 
>> "Since one TLB inner-shareable invalidation is enough to avoid this erratum, the number of injected TLB invalidations should be minimized in the trap handler to mitigate the performance impact due to this workaround."
>> 
>> This PR introduces a mechanism to defer instruction cache (ICache) invalidation for AArch64 to address the Arm Neoverse N1 erratum 1542419, which causes significant performance overhead if ICache invalidation is performed too frequently. The implementation includes detection of affected Neoverse N1 CPUs and automatic enabling of the workaround for relevant Neoverse N1 revisions.
>> 
>> Changes include:
>> 
>> * Added a new diagnostic JVM flag `NeoverseN1Errata1542419` to enable or disable the workaround for the erratum. The flag is automatically enabled for Neoverse N1 CPUs prior to r4p1, as detected during VM initialization.
>> * Introduced the `ICacheInvalidationContext` class to manage deferred ICache invalidation, with platform-specific logic for AArch64. This context is used to batch ICache invalidations, reducing performance impact. As the address for icache invalidation is not relevant, we use the nmethod's code start address.
>> * Provided a default (no-op) implementation for `ICacheInvalidationContext` on platforms where the workaround is not needed, ensuring portability and minimal impact on other architectures.
>> * Modified barrier patching and relocation logic (`ZBarrierSetAssembler`, `ZNMethod`, `RelocIterator`, and related code) to accept a `defer_icache_invalidation` parameter, allowing ICache invalidation to be deferred and later performed in bulk.
>> 
>> Testing results: linux fastdebug build
>> - Neoverse-N1 (Graviton 2)
>>    - [x] tier1: passed
>>    - [x] tier2: passed
>>    - [x] tier3: passed
>>    - [x] tier4: 3 failu...
>
> Evgeny Astigeevich has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix linux-cross-compile riscv64 build

src/hotspot/share/runtime/icache.hpp line 139:

> 137: class DefaultICacheInvalidationContext : StackObj {
> 138:  private:
> 139:   NONCOPYABLE(DefaultICacheInvalidationContext);

Not a review, just a drive-by comment. @xmas92 suggested moving the `NONCOPYABLE` to the
private part of the class, as a style issue. It used to be that `NONCOPYABLE` was best used in the
private part of a class, because of how it was implemented.  But with the change to using deleted
definitions, it's actually better to have it in the public part. That way you get an "attempt to use a
deleted function" error rather than possibly getting an "attempt to use an inaccessible function"
error.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/28328#discussion_r2657751265