[vbox-dev] 386, 486 or 686?

Klaus Espenlaub klaus.espenlaub at oracle.com
Mon Jun 18 09:36:54 PDT 2012

On 18.06.2012 17:56, Ribhi Kamal wrote:
> I'm worried that some of these relatively new instructions might add
> more instructions that are troublesome to virtualize. So I'm inclined to
> using the arch with the smallest feature set, 386, rather than 686. Does
> that sound right?

You expect detailed answers without giving the minimal set of facts - 
what virtualization mode are you talking about? VirtualBoxe effectively 
has 3 of them - the recompiler, raw mode and hardware virtualization. In 
the last two effectively all "normal" instructions are executed usually 
without significant overhead (ignoring interrupts/faults or unrelated 
instructions which need attention by the hypervisor). In general, 
VirtualBox tries to stay as much as possible in those two virtualization 
modes (if available - there are conditions which may force going to the 

In any case, VirtualBox doesn't offer instructions which the host CPU 
doesn't have (and actually might disable the CPUID feature bits for some 
instruction set extensions which the CPU might have), and thus there is 
no need for completely emulating something in software (the recompiler 
often does it nevertheless, just to have everything under control).

So don't worry too much about those fuzzy "relatively new instructions".


> On Mon, Jun 18, 2012 at 10:58 AM, Alexey Eromenko <al4321 at gmail.com
> <mailto:al4321 at gmail.com>> wrote:
>     On Mon, Jun 18, 2012 at 5:51 PM, Ribhi Kamal <rbhkamal at gmail.com
>     <mailto:rbhkamal at gmail.com>> wrote:
>      > >From a Virtualbox point of view, would a Linux kernel be
>     easier/faster
>      > to virtualize if it was targeted for a 386 CPU architecture, 486
>     or 686?
>      > Would it make a difference at all?
>     I think 686 would be faster, as more instructions are available
>     (MMX, cmov, ...)
>     --
>     -Alexey Eromenko "Technologov"
>     _______________________________________________
>     vbox-dev mailing list
>     vbox-dev at virtualbox.org <mailto:vbox-dev at virtualbox.org>
>     https://www.virtualbox.org/mailman/listinfo/vbox-dev
> --
> -- Ribhi
> _______________________________________________
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