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

[Nick Gasson]

On Wed, 4 Nov 2020 08:30:41 GMT, Nick Gasson <ngasson 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.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.

Here is the patch prints CPU features for all CPUs, backported to 3.10:
https://patchwork.kernel.org/project/linux-arm-kernel/patch/20150209083040.217202212@linuxfoundation.org/

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