Best practices for I/O APIC, PAE/NX, VT-x, Nested Paging

[socratis]

When I setup a new VM, I usually start with the VBox template. But then I almost always enable I/O APIC, PAE/NX, VT-x and Nested Paging, no matter what the template says.

Is there a potential problem with having all of them enabled? I know that there are some guests OSes that have to have some or all of them enabled. The question is if there are any guest OSes that have to have some of them disabled.

[noteirak]

Interesting question...
I know I always enable them as well, and I never came accross an issue. I never installed any old OS tho. The oldest system installed in a VM for me is Win 2003/XP.

[mpack]

There is no best practice. These are simply a description of the motherboard features, and there is no "correct" motherboard. It may be true however to say that newer operating systems (Win7 and later, recent Linux's) will expect modern motherboards and hence will not be well tested on legacy chipsets. OTOH older operating systems may not be able to use (say) IO-APIC efficiently.

The templates are chosen by experts to be most suitable for the selected guest. It's generally IMHO best to stick with the template (*) unless you know otherwise.

(*) Some options like RAM size (not VRAM) and disk size are chosen to be conservative, rather than compatible. You can change these if you know you have plenty of resources.

[socratis]

But you see, that's the issue that prompted my question. I was trying to recreate a VM in order to reply to a user's question and in the recipe for Win2008 (that's a really recent server), it proposes 512 RAM, no I/O APIC and no PAE/NX. If you try the Win7 template, it proposes 512 RAM, no I/O APIC, no PAE/NX. That doesn't sound right. Don't get me started that all of the templates have the boot from floppy(?) option as default! Thank $(Deity) they don't have the 5"1/4 option

[mpack]

I think they expect "modern" to be 64bit, in which case the template will definitely enable IO APIC.

It's possible to configure a 5 1/4" floppy, though not in the GUI.

[michaln]

But you see, that's the issue that prompted my question. I was trying to recreate a VM in order to reply to a user's question and in the recipe for Win2008 (that's a really recent server), it proposes 512 RAM, no I/O APIC and no PAE/NX. If you try the Win7 template, it proposes 512 RAM, no I/O APIC, no PAE/NX. That doesn't sound right.

As mpack said, this is for 32-bit guests. The defaults are chosen to provide reasonable performance even without hardware virtualization.

The general strategy for the defaults is to provide the minimum hardware features the guest OS requires. The reason is that emulating the "extras" can be very expensive and hurt performance. On the other hand, turning everything on rarely improves performance even on hosts with good hardware virtualization support (esp. nested paging).

Note that this does not apply to the RAM/disk sizes. Again those are reasonable minimums but it's really up to the user to decide how much the guest needs and the host can spare. VirtualBox can't make very good guesses because it has no information about how many VMs might be running simultaneously or what other load the host needs to handle.

Don't get me started that all of the templates have the boot from floppy(?) option as default!

But do they actually create floppy drives for the VMs? I don't think so...

[Ramshankar]

As far as "VT-x, Nested Paging" goes, if your CPU supports it, having Nested Paging enabled is always recommended. It means VirtualBox doesn't have to intercept guest page-faults and intercepts cause significant overhead.

[socratis]

That's good to know Ramshankar.

I did a little more digging and went searching in the source code to see what are the suggested values for each OS and version. My source was http://www.virtualbox.org/svn/vbox/trunk/src/VBox/Main/src-all/Global.cpp. I can post an easier to read CSV or ODS version.