Re: [PATCH 0/5] Split the emulator: decode & execute

Avi Kivity
2007-09-17T17:29:21Z

Laurent Vivier wrote:
> Avi Kivity wrote:
>
>> Laurent Vivier wrote:
>>
>>> Avi Kivity wrote:
>>>
>>>
>>>> Laurent Vivier wrote:
>>>>
>>>>
>>>>> These patches split the emulator in two parts: one to decode the
>>>>> instruction,
>>>>> the other to execute it. The decode part is then called only when needed.
>>>>>
>>>>>
>>>>>
>>>>>
>>>> Patchset looks good, but fails booting FC6 x86-64 on Intel. It may be a
>>>> merge error (did not apply cleanly due to other changes). I pushed this
>>>> as a 'split-emulator' branch on the kvm.git repository.
>>>>
>>>>
>>>>
>>> I think I found the bug (not a merge error...): I just supposed that an
>>> instruction fetch cannot failed.
>>>
>>>
>>>
>> Interesting. I don't see how an instruction fetch can fail on
>> uniprocessor. Can you give details of the failure?
>>
>> Instruction fetches can fail on SMP so a fix is certainly needed.
>>
>
> OK, I spoke too fast.
>
> x86_decode_insn() fails because it is not able to decode:
>
> 0xffffffff8110b7ef <__copy_user_nocache+47>: movnti %r11,(%rdi)
> or
> 0xffffffff8110b7ef <__copy_user_nocache+47>: 0x4c 0x0f 0xc3 0x1f
>
> 0x4c is decoded as a REX prefix.
> 0x0f is decoded as a Two-byte opcode
> but 0xc3 is unknown in twobyte_table, so we exit because of an unrecognized
> opcode ("Cannot emulate").
>
>

Not being able to emulate is sometimes legitimate. In the case of
writing to a write-protected guest page table, we simply
un-write-protect it and go back to the guest (which should now execute
the instruction natively).

Perhaps the logic that deals with this (the call to
kvm_mmu_unprotect_page_virt() in emulate_instruction()) was broken by
your changes.

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