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

[Avi Kivity <avi@redhat.com>]

On 04/08/2010 05:16 PM, Marcelo Tosatti wrote:
> On Thu, Apr 08, 2010 at 11:05:56AM +0300, Avi Kivity wrote:
>    
>> 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.
>>      
> And new ioctl to save/restore save_iopl/save_vm.
>    

That ioctl already exists, KVM_{GET,SET}_REGS.

We're writing via KVM_SET_SREGS eflags.vm=1 and eflags.iopl=3 while 
cr0.pe=0.  vmx_set_rflags() notices this and sets rmode.save_vm=1 and 
rmode.save_iopl=3.  Next we write via KVM_SET_SREGS cr0.pe=1.  So we 
call enter_pmode(), and recover eflags.vm and eflags.iopl from rmode.vm 
and rmode.iopl.  Win!

It's similar to how we handle cr0.ts, sometimes the host owns it so we 
keep it in a shadow register, sometimes the guest owns it so we keep it 
in cr0.

>>>> 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. :)
>>>        
> Do you mean you'd be interested in writing the patch? Sure, go ahead,
> let me know otherwise.
>    

I took it to mean he wants to kill the other qemu workarounds for kernel 
bugs.

-- 
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