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

Gleb Natapov
2010-02-14T16:53:21Z

On Sun, Feb 14, 2010 at 05:37:39PM +0100, Jan Kiszka wrote:
> 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.
>
I see. And I see why it worked, but it shouldn't have been working for
SVM. I prefer to look for general solution here that works for SVM/VMX.

--
Gleb.