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 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.

> problem is that SVM supports only the first way. Intel advised us to use
> reinjection mechanism, so that what we use on VMX, but since migration
> can happen from Intel to AMD and vice versa we chose to reexecute
> instruction after migration on those rare occasions that migration
> happens exactly after intercepted soft i/e.

Jan


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