Re: [PATCH] KVM: x86: Add instruction length to VCPU event state

[Jan Kiszka <jan.kiszka@web.de>]

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Gleb Natapov wrote:
> On Sat, Feb 13, 2010 at 08:20:41PM +0100, Jan Kiszka wrote:
>> Gleb Natapov wrote:
>>> On Sat, Feb 13, 2010 at 07:41:35PM +0100, Jan Kiszka wrote:
>>>> Gleb Natapov wrote:
>>>>> On Sat, Feb 13, 2010 at 06:49:44PM +0100, Jan Kiszka wrote:
>>>>>> Gleb Natapov wrote:
>>>>>>> On Sat, Feb 13, 2010 at 10:51:40AM +0100, Jan Kiszka wrote:
>>>>>>>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>>>>>>>
>>>>>>>> VMX requires a properly set instruction length VM entry field when
>>>>>>>> trying to inject soft exception and interrupts. We have to preserve this
>>>>>>>> state across VM save/restore to avoid breaking the re-injection of such
>>>>>>>> events on Intel. So add it to the new VCPU event state.
>>>>>>>>
>>>>>>> We shouldn't re-inject soft exceptions/interrupts after migration, but
>>>>>>> re-execute instruction instead. Instruction length field doesn't exist
>>>>>>> on SVM and migration shouldn't expose implementation details.
>>>>>>>
>>>>>> Hmm, then I guess this totally untested patch should fly:
>>>>>>
>>>>> I don't understand what problem are you trying to solve by your patch.
>>>>> During normal operation event_exit_inst_len will be set to correct
>>>>> value. After migration rip will point to int instruction an no even will
>>>>> be pending at all. Here is the patch:
>>>> The patch will cause an endless loop if BP interception is enabled.
>>>>
>>> How? This code path is not executed normally.
>> Oh, I read it the other way around, but it is supposed to mask soft
>> exceptions/irqs (clearing *.injected is missing then).
>>
>>>> What is the purpose of keeping event_exit_inst_len around? Either we
>>>> need it also across user space exists, then we have to save/restore or
>>>> reconstruct it, or we don't need it, then simply drop it.
>>>>
>>> Why we need to save/restore is if we need it across user space exits?
>>> We need to save/restore it only if we nedd it across migration.
>>>
>>> When exception happens during soft interrupt/exception delivery soft i/e
>>> should be retried somehow. There are two ways to do that. First one is just
>>> reenter guest with the same rip. Instruction will be reexecuted and
>>> event redelivered. Another is to reinject event via event reinjection
>>> mechanism and for that we need to tell CPU how to calculate rip of a next
>>> instruction and this is done by providing event_exit_inst_len. The
>> But I still fail to see the case where event_exit_inst_len is set to
>> anything but 1 or 2 and where it is related to anything else than exits
>> at INT3, INT X, or INTO.
>>
> You can't know real instruction length without decoding it or relying on
> VMX exit info. What if prefix were used for INT X?
> 

OK, makes sense now. Then let's go with your suggestion, will post it as
a patch.

Thanks for explaining,
Jan


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