Re: VMX and save/restore guest in virtual-8086 mode

[Avi Kivity <avi@redhat.com>]

On 04/08/2010 10:54 AM, Jan Kiszka wrote:
>
>    
>>>> Looks like KVM_SET_REGS should write rmode.save_iopl (and a new save_vm)?
>>>>
>>>>          
>>> Just like we manipulate the flags for guest debugging in the
>>> set/get_rflags vendor handlers, the same should happen for IOPL and VM.
>>> This is no business of enter_pmode/rmode.
>>>
>>>        
>> This is vendor specific code, and it isn't manipulating guest values,
>> only host values (->set_rflags() is called when the guest value changes,
>> which isn't happening here).  Of course some refactoring will be helpful
>> here.
>>      
> Actually, the bug is that enter_pmode/rmode update save_iopl (and that
> no one saves the VM bit). That should happen in vmx_set_rflags to also
> keep track of changes _while_ we are in rmode.

Exactly - that's what I suggested above.

> enter_rmode/pmode should
> just trigger a set_rflags to update things.

Not what I had in mind, but a valid implementation.

> And vmx_get_rflags must
> properly inject the saved flags instead of masking them out.
>    

Yes.  No one ever bothers to play with iopl in real mode, so we never 
noticed this.  We do this for cr0 for example.

>> It's a bugfix that can go into -stable and supported distribution kernels.
>>      
> Well, would be happy to throw out tones of workaround based on this
> approach. :)
>    

And I'll be happy to apply such patches.  Just ensure that 2.6.32.y and 
above have the fixes so we don't introduce regressions (I think most 
workarounds are a lot older).

-- 
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