Re: What happens on an INT80 instruction

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Cameron Macdonell wrote:
> Hi,
>
> I'm trying to understand guest virtualization at the lower levels.  I  
> have a somewhat basic question:  How does KVM virtualize an int80  
> instruction from a guest?  A pointer to an answer is just as good as  
> an answer itself.
>   

The same thing happens as it does on normal hardware.

The way VT/SVM works (at a high level), is that certain instructions and 
events check a special area called the VMCS/VMCB to determine whether 
the event should generate a vmexit which is really just a special type 
of trap.

There are no hooks for interrupts 32-255 so the hardware operates as it 
normally would.  If you're interested in getting a trap for int80 within 
KVM, you'll have to trap sidt/lidt and virtualize the IDT.  You'll need 
to setup a fake IDT and have the int80 handler do a hypercall.  This is 
complicated if the guest is using a fast-syscall mechanism.  It may be a 
little challenging finding a piece of guest memory to take over that has 
a valid virtual mapping.  To solve this in the general case, you'll need 
to have the guest be aware of a memory hole.  If you can limit yourself 
to things like Linux and Windows, you can probably just rely on some 
memory within the BIOS area (both Linux and Windows always have valid 
mappings of the BIOS memory).

If you need to enforce that int80s go to you, you'll need to 
write-protect this memory too.

Regards,

Anthony Liguori

> Thanks,
> Cam
>
>
>
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