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

Gleb Natapov
2010-02-14T11:15:46Z

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? How about disabling #BP intercept and reenter guest (may
be single step it at this point to reenable #BP interception ASAP)?

PS: moving rip after int3 may lead to wrong rip to be pushed on
exception stack if exception happens during event delivery.

--
Gleb.