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

[Gleb Natapov <gleb@redhat.com>]

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?

--
			Gleb.