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

[Avi Kivity <avi@redhat.com>]

On 04/08/2010 10:22 AM, Jan Kiszka wrote:
> Avi Kivity wrote:
>    
>> On 04/07/2010 11:24 PM, Marcelo Tosatti wrote:
>>      
>>> During initialization, WinXP.32 switches to virtual-8086 mode, with
>>> paging enabled, to use VGABIOS functions.
>>>
>>> Since enter_pmode unconditionally clears IOPL and VM bits in RFLAGS
>>>
>>>           flags = vmcs_readl(GUEST_RFLAGS);
>>>           flags&= ~(X86_EFLAGS_IOPL | X86_EFLAGS_VM);
>>>           flags |= (vmx->rmode.save_iopl<<   IOPL_SHIFT);
>>>           vmcs_writel(GUEST_RFLAGS, flags);
>>>
>>>
>>>        
>>
>> 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.

>>> The following patch fixes it, but it has some drawbacks:
>>>
>>> - cpu_synchronize_state+writeback is noticeably slow with tpr patching,
>>>     this makes it slower.
>>>
>>>        
>> Isn't it a very rare event?
>>      
> It has to be - otherwise the decision to go for full sync and individual
> get/set IOCTL would have been wrong. What happens during tpr patching?
>
>    

tpr patching listens for instructions which access the tpr and patches 
them to a call instruction (targeting some hacky code in the bios).  
Since there are a limited number of such instructions (20-30 IIRC) you 
expect tpr patching to happen very rarely.

>>> - Its a fugly workaround.
>>>
>>>        
>> True.
>>
>>      
> Still likely the way to go for old kernels.
>
>    

It's a bugfix that can go into -stable and supported distribution kernels.

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