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After installing Windows XP on to a VirtualBox VM instance configured with multiple virtual processors, I noticed that the guest's support for ACPI S3 and S4 is lacking, though at least some other ACPI functionality is working properly (such as MP enumeration). Why is this so? (I suspect the answer is because the ACPI tables are incomplete.)

Is there a way to make guest ACPI S3 / S4 supported in VirtualBox for a more seamless experience? If ACPI S3 / S4 is unsupported, what is the rationale for the acpisleepbutton in VBoxManage controlvm?

What technical reasons, if any, are preventing S4 from working properly in the guest / showing up as an option? It seems to me that S4 should be trivial to implement, as opposed to S3, much like how S4 support is easier to implement than S3 in Linux - it requires minimal hardware support, to the extent that some call it "software suspend".

When attempting S3 or S4 from Linux, a similar situation arises. /sys/power/state offers neither disk or mem. Again, I have to wonder why; S4 (disk) is driven mostly through software and has minimal hardware requirements.

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