Re: Question about EMULATE_DO_MMIO

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

Nitin A Kamble wrote:
> Hi Avi,
>    I was trying to avoid vm86 mode completely, by using emulation for
> all the real mode code emulation. It is doing better than vm86 mode, 

Why do you think it is better?  I would have thought that vm86 is much
faster.

I thought of doing it this way:

- try to run normally
- if vm entry failed due to invalid guest state (the famous 0x80000021),
then emulate 50 instructions
- repeat

That will also handle the few cases where VT can't handle protected mode.

> but
> I am seeing one issue with this approch.
>
>    The emulator is failing due to this code. Seems like the execution
> need to go back to qemu to finish some pending MMIO.
>
>        if (vcpu->mmio_is_write) {
>                 vcpu->mmio_needed = 0;
>                 return EMULATE_DO_MMIO;
>         }
>
> Do you have how should it be handled? I am not clear what needs to be
> done to handle this situation properly.
>   

It should exit to userspace, same as the non-emulating case.  However it
shouldn't happen for normal writes.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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