Re: What happens on an INT80 instruction

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Anthony Liguori wrote:
> Jun Koi wrote:
>   
>> On 10/1/07, Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org> 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.
>>>
>>> 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.
>>>     
>>>       
>> This is a bit vague to me. Why do you need "a piece of guest memory" here?
>>   
>>     
>
> You don't just need guest memory, you need a valid guest virtual address 
> too.  The IDTR contains a guest VA.  If you want to create your own IDT, 
> then it has to be a valid VA in the guest's address space.
>
>   

You can set the guest idt size to zero and trap the double fault exception.


-- 
error compiling committee.c: too many arguments to function


-------------------------------------------------------------------------
This SF.net email is sponsored by: Microsoft
Defy all challenges. Microsoft(R) Visual Studio 2005.
http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/