RFR: 8264543: Cross modify fence optimization for x86

[Xubo Zhang]

On Thu, 27 May 2021 22:59:11 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Hi Gerard,
>> Although we don't have benchmark data yet, serialize is the recommended alternative vs. using CPUID. Also, in the virtual machine environment, serialize provides full/fast serialization without causing VMEXIT
>
>> Although we don't have benchmark data yet, serialize is the recommended alternative vs. using CPUID. 
> 
> I don't see any rush to make this change so there is time to gather the missing benchmark data. If Intel recommend making this change then we should definitely consider it, but we need to know that it won't adversely affect anyone first. And as Kim said this should be done for all x86 in that case.
> 
>> Also, in the virtual machine environment, serialize provides full/fast serialization without causing VMEXIT
> 
> Sorry what do you mean by this?
> 
> Thanks,
> David

@dholmes-ora VM exit is transitioning from a running VM to the hypervisor. The processor has to save the state of the VM. It is a costly operation. The cpuid instruction requires VM exit (https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-vol-3c-part-3-manual.pdf  25.1.2)

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PR: https://git.openjdk.java.net/jdk/pull/3334