RFR: 8224974: Implement JEP 352

[Andrew Dinn]

On 31/07/2019 16:46, Aleksey Shipilev wrote:
> On 7/31/19 4:46 PM, Andrew Dinn wrote:
>> Well, the failure happened during the build process so I didn't
>> (couldn't) debug it. I disabled the assert in supports_cpuflush() in
>> order to allow the build to complete and then ran up the resulting JVM
>> inside gdb to check what was going on. The problem is that with my patch
>> supports_cpuflush() is called from Assembler::clflush() and the latter
>> is called from icache_x86.cpp very early during bootstrap (I think this
>> is neded to flush the flush routine used to flush the code cache).
>> Anyway, the call happens so early that it precedes the call to
>> VM_Version::get_processor_features which sets up the _features mask
>> tested by the assert.
> 
> I believe you can untie this bootstrapping circularity by relaxing the assert with
> Universe::is_fully_unitialized(). It still gives us window to fail with SIGILL if the stub is called
> early during bootstrap, but that would be something to fix *if* we ever find ourselves there.
Yes indeed, that seems like by far the cleanest solution. The only other
alternative is to move the call to VM_Version::initalize earlier in the
Universe bootstrap sequence and I doubt anyone wants to risk that.

So, now I have

#ifdef _LP64
  static bool supports_clflush() {
    // clflush should always be available on x86_64
    // if not we are in real trouble because we rely on it
    // to flush the code cache.
    // Unfortunately, Assembler::clflush is currently called as part
    // of generation of the code cache flush routine. This happens
    // under Universe::init before the processor features are set
    // up. Assembler::flush calls this routine to check that clflush
    // is allowed. So, we give the caller a free pass if Universe init
    // is still in progress.
    assert ((!Universe::is_fully_initialized() || (_features &
CPU_FLUSH) != 0), "clflush should be available");
    return true;
  }
  . . .

which seems to work ok.

I'll post a new webrev when I have handled Brian's comments and also any
other feedback you may still have on my last reply.

regards,


Andrew Dinn
-----------
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