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

[Andrew Dinn]

On Wed, 4 Nov 2020 16:46:36 GMT, Andrew Dinn <adinn at openjdk.org> wrote:

>> 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.

@theRealAph BTW, I think Nick is right that this patch is not needed. Are you ok to reject it?

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