RFR: JDK-8158502: aarch32: detect QEMU user-mode emulation

[Alex Kashchenko]

Hi,

On 06/02/2016 10:03 PM, Alex Kashchenko wrote:
 >> On 06/02/2016 07:57 PM, Edward Nevill wrote:
 >> [...]
>> The reading of /proc/cpuinfo is a horrible wart which we should try to
>> expunge. getauxv() is only marginally better. Both are non portable and
>> break under virtualisation.
>>
>> The only reliable way to determine what features the CPU has is to test
>> the CPU itself. Unfortunately ARM have a habit of making the ID
>> registers only accessible in privileged modes, presumably to allow for
>> virtualisation, but then the kernels don't pass the information through.
>>
>> I would almost prefer a scheme where we test an instruction from the
>> feature set we wish to use and if we catch a SIGILL we mark the feature
>> as not available.
>
> Thanks for the explanation. I know that OpenSSL library uses this
> approach [1]. I also checked Android cpufeatures library [2] (that is
> likely fine-tuned for arm32), but unfortunately it doesn't use this
> approach [3], probably because it runs in more controlled environment.

I prototyped a simplistic utility for CPU capabilities probing - 
http://cr.openjdk.java.net/~akasko/aarch32/jdk8u/8158502/detect.01/

It uses sigaction and siglongjmp for probing, probe-functions are 
precompiled. Currently it can only detect ARMv6T2 using movt, but adding 
more probes should be straightforward.

Looking for comments whether something like this can be used in 
vm_version_aarch32.


References:
  - 
https://github.com/openssl/openssl/blob/4ac0329582829f5378d8078c8d314ad37db87736/crypto/armcap.c#L87
  - https://lkml.org/lkml/1996/4/27/27

-- 
-Alex