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

[Jan Kiszka <jan.kiszka@siemens.com>]

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.

Not need. The information will all be contained in eflags and cr0 as
returned to userspace. The bug is that the wrong information is
currently returned, thus saved/restored.

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

ATM, I fighting against too many customer bugs. And you have the test
case for this particular issue, I assume. So don't wait for me here.

Jan

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