RFR: JDK-8201483 Make it possible to disable JVM features

[Magnus Ihse Bursie]

On 2018-04-12 23:30, David Holmes wrote:
> On 12/04/2018 11:33 PM, Magnus Ihse Bursie wrote:
>> On 2018-04-12 14:15, David Holmes wrote:
>>> Hi Magnus,
>>>
>>> On 12/04/2018 9:39 PM, Magnus Ihse Bursie wrote:
>>>> It is currently easy to add new JVM features to the JVM build, but 
>>>> it is not possible to remove features.
>>>>
>>>> With this change, features can be both added or removed from the 
>>>> default set. They are added using --with-jvm-features=f1,f2 and 
>>>> removed using --with-jvm-features=-f1,-f2. The syntax can be 
>>>> combined, so --with-jvm-features=dtrace,-nmt will enable dtrace but 
>>>> disable native memory tracking.
>>>
>>> I need to point out that we have never tested disabling individual 
>>> VM features likes this. They are either all on, or all off for the 
>>> minimal VM! There may be implicit dependencies between features.
>>
>> Well, I have. :-) However, I don't do that regularly, and changes 
>> might very well have crept in. As always, if you build something 
>> non-standard that is not regularly tested, you're on your own.
>
> Feels to me like you've taken away the safety-fence and are 
> encouraging people to attempt these unsupported configurations. 
> Whether that was your intent or not.

It is always possible to configure something that does not work. :-) We 
can make it more easy to do the right thing, but if we were to make 
impossible everything that has not been tested, then we would also make 
things impossible that are needed for some use cases.

Wrt JVM features, this has *always* been possible. If you use 
"--with-jvm-variants=custom --with-jvm-features=jvmti,nmt" then you 
*are* going to build an almost (or fully?) useless JVM. In fact, this 
method made it *much* harder to try to get a functioning JVM without a 
specific feature.

I think I should update the build documentation regarding JVM features 
though, and I can definitely add some wisely worded warnings about 
unsupported combinations of features, especially if removing them.

>
>> In any case, the purpose of this is not so much to disable existing 
>> JVM features (after all, no one has really been missing this 
>> functionality), as to pave the way for the upcoming patch for 
>> including/excluding individual GCs.
>
> Surely a GC selection flag would have sufficed.
It was the common agreement of both the build team and the GC folks 
responsive for the upcoming selectively GC inclusion patch, that this 
was better handled as JVM features than a separate GC flag.

/Magnus
>
> David
>
>> /Magnus
>>
>>>
>>> David
>>>
>>>> I also included some additional code cleanup and fixes, such as 
>>>> printing the JVM feature set at the summary.
>>>>
>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8201483
>>>> WebRev: 
>>>> http://cr.openjdk.java.net/~ihse/JDK-8201483-disable-JVM-features/webrev.01 
>>>>
>>>>
>>