[vbox-dev] why should virtualbox 5.1.4 enable uses 32-bit capabilities (legacy support in use) as default?

[getonga]

I use virtualbox as the vagrant provider, it works very well.
Recently I update the virtualbox to version 5.1.4, the message appears:
$ dmesg | grep -i warn
[   92.028562] capability: warning: `VirtualBox' uses 32-bit capabilities (legacy support in use)


I have three linux machines, all has these msg. All of them have the secure virtual mode enable. Why should virtualbox use 32-bit as default, not the 64-bit? Some old versions not use this as default, but I am not sure which version starts to use this as default.


It also works, but I'd like to know why this enables as default?
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