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

Gleb Natapov
2010-02-14T14:45:03Z

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.

--
Gleb.