Re: [PATCH] KVM: VMX: Update instruction length on intercepted BP

Jan Kiszka
2010-02-14T16:37:58Z

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Gleb Natapov wrote:
> On Sun, Feb 14, 2010 at 12:39:14PM +0100, Jan Kiszka wrote:
>> Gleb Natapov wrote:
>>> On Sun, Feb 14, 2010 at 11:47:58AM +0100, Jan Kiszka wrote:
>>>> Gleb Natapov wrote:
>>>>> On Sun, Feb 14, 2010 at 11:26:31AM +0100, Jan Kiszka wrote:
>>>>>> Gleb Natapov wrote:
>>>>>>> On Sat, Feb 13, 2010 at 10:31:12AM +0100, Jan Kiszka wrote:
>>>>>>>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>>>>>>>
>>>>>>>> We intercept #BP while in guest debugging mode. As VM exists due to
>>>>>>>> intercepted exceptions do not necessarily come with valid
>>>>>>>> idt_vectoring, we have to update event_exit_inst_len explicitly in such
>>>>>>>> cases. At least in the absence of migration, this ensures that
>>>>>>>> re-injections of #BP will find and use the correct instruction length.
>>>>>>>>
>>>>>>> event_exit_inst_len is only used for event reinjection. Since event
>>>>>>> intercepted here will not be reinjected why updating event_exit_inst_len
>>>>>>> is needed here?
>>>>>> In guest debugging mode a #BP exception is always reported to user space
>>>>>> to find out what caused it. If it was the guest itself, the exception is
>>>>>> reinjected, on older kernels via KVM_SET_GUEST_DEBUG and since 2.6.33
>>>>>> via KVM_SET_VCPU_EVENTS (the latter requires some qemu patch that I will
>>>>>> post later).
>>>>>>
>>>>>> As we currently do not update event_exit_inst_len on #BP exits,
>>>>>> reinjecting fails unless event_exit_inst_len happens to be 1 from some
>>>>>> other exit.
>>>>>>
>>>>> Hmm, how does it work on SVM then where we do not have
>>>>> event_exit_inst_len so execution will resume on the same rip that caused
>>>>> #BP after event reinjection?
>>>>>
>>>> Maybe not at all. I don't think I've tested this scenario on amd so far.
>>>> Guess it needs some special handling in svm to move rip after the int3
>>>> when requesting to inject #BP.
>>>>
>>> This will work for VMX too, no? So may be we should design something
>>> that will work for both VMX and SVM before applying patches that make
>>> oly VMX work?
>> VMX used to work, so my patch is actually a regression fix. I bet this
>> was accidentally broken while cleaning up the interrupt handling of VMX.
>>
> VMX used to always reexecute instruction.

...since 66fd3f7f90. And that was what broke this guest debugging corner
case.

Jan

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