Re: What happens on an INT80 instruction

[Cam Macdonell <cam-edFDblaTWIyXbbII50Afww@public.gmane.org>]

Anthony Liguori wrote:
> Cam Macdonell wrote:
>> Anthony Liguori wrote:
>>> 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.
>>>
>>
>> Thanks Anthony.  Does an int80 from an application in the guest always 
>> cause a vmexit (in kvm's case at least)?
> 
> No, an int80 would never generate a trap in KVM.  The only way to make 
> it generate a trap is for an int80 to trigger some other event that 
> would generate a trap.  This is what I meant by taking over the guest's 
> IDT such that you could change the int80 handler to do a hypercall.
> 
> I presume you're looking into doing a guest IDS right?
> 

Actually, I looking into doing a PhD dissertation :)  I'm just trying to 
get a better working understanding of how kvm (and other VMMs) handle 
instructions like int80 that should trap into the OS, but of course in a 
VM need to trap into the guest OS (which is running at user-level) and 
not the host OS.  Do traps by a guest app to the guest OS involve the 
VMM at all?

Pardon my ignorance, what is IDS?

Thanks,
Cam

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