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

[Andrew Dinn]

On Wed, 4 Nov 2020 12:13:55 GMT, John Paul Adrian Glaubitz <glaubitz at openjdk.org> wrote:

>> Tested slowdebug build on Windows+Arm64 with this patch, and smoked tested it with `jtreg:tier1_compiler_1` successfully.
>> 
>> Change itself looks go to me too (but I'm not a reviewer).
>
> Does this account for the fact that CPU cores can be offline?
> 
> Normally, the number of available cores is determined with ```sysconf()``` in a portable manner and ```sysconf()``` differentiates between _configured_ and _online_ processors:
> 
>         - _SC_NPROCESSORS_CONF
>               The number of processors configured.  See also get_nprocs_conf(3).
> 
>         - _SC_NPROCESSORS_ONLN
>               The number of processors currently online (available).  See also get_nprocs_conf(3).
> 
> The number of online processors can be lower than the number of configured processors.
> 
> I remember fixing an issue regarding this feature in PulseAudio as the testsuite broke on SPARC on Linux, see: https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/commit/1df21e6ab6cd42e2f7601a6c5577c20b7e3d1046

> And while we're talking about Linux, can we really not get the info we need
without parsing /proc/cpuinfo? And do we need to parse the entire file?

At present reading /proc/cpuinfo that is the only reliable way I know of to identify whether dcpop is  a supported feature (used to force persistence of data to memory). That is needed to support use of NVRAM-backed MappedByteBuffers.

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