RFR: 8255799: AArch64: CPU_A53MAC feature may be set incorrectly

[Nick Gasson]

On Tue, 3 Nov 2020 16:58:58 GMT, Anton Kozlov <akozlov at openjdk.org> wrote:

> JDK-8255716 (#983) uncovered that os::processor_count on Linux can be not equal to number of cores reported in /proc/cpuinfo. The latter historically was used to decide CPU_A53MAC feature. This patch restores feature detection based on /proc/cpuinfo.
> 
> Testing: linux -version
> Unfortunately I cannot test windows/aarch64, CC: @lewurm @luhenry

src/hotspot/cpu/aarch64/vm_version_aarch64.cpp line 186:

> 184: 
> 185:   if (_cpu == CPU_ARM && (_model == 0xd07 || _model2 == 0xd07)) _features |= CPU_STXR_PREFETCH;
> 186:   // If max number of cores (on Linux reported in /proc/cpuinfo) then if _model is an A57 (0xd07)

Should be "if max number of cores = 1"?

src/hotspot/cpu/aarch64/vm_version_aarch64.hpp line 43:

> 41:   static int _stepping;
> 42: 
> 43:   // Used to decide CPU_A53MAC feature of some single-core CPUs. Note this

It's not a feature of single-core CPUs: AFAIK it's a work around for very old arm64 kernels that only reported a single CPU in `/proc/cpuinfo` on multi-core systems where you may have a mix of different CPU types (i.e. mixed A53/A57 where the A57 is reported in cpuinfo).

I wonder if we should just remove this workaround altogether? The patch to list all CPUs in `/proc/cpuinfo` was backported to at least the 3.10 series. I really doubt there's anyone running latest OpenJDK on a A53 with such an old kernel.

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PR: https://git.openjdk.java.net/jdk/pull/1039