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

Jan Kiszka
2010-02-14T10:48:09Z

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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.

Jan

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