Re: [Qemu-devel] KVM: Windows 64-bit troubles with user space irqchip

[Gleb Natapov]

On Wed, Feb 02, 2011 at 04:52:11PM +0100, Jan Kiszka wrote:
> On 2011-02-02 16:46, Gleb Natapov wrote:
> > On Wed, Feb 02, 2011 at 04:35:25PM +0100, Jan Kiszka wrote:
> >> On 2011-02-02 16:09, Avi Kivity wrote:
> >>> On 02/02/2011 04:52 PM, Jan Kiszka wrote:
> >>>> On 2011-02-02 15:43, Jan Kiszka wrote:
> >>>>>  On 2011-02-02 15:35, Avi Kivity wrote:
> >>>>>>  On 02/02/2011 04:30 PM, Jan Kiszka wrote:
> >>>>>>>  On 2011-02-02 14:05, Avi Kivity wrote:
> >>>>>>>>   On 02/02/2011 02:50 PM, Jan Kiszka wrote:
> >>>>>>>>>>>
> >>>>>>>>>>    Opps, -smp 1. With -smp 2 it boot almost completely and then 
> >>>>>>>>>> hangs.
> >>>>>>>>>
> >>>>>>>>>   Ah, good (or not good). With Windows 2003 Server, I actually get 
> >>>>>>>>> a Blue
> >>>>>>>>>   Screen (Stop 0x000000b8).
> >>>>>>>>
> >>>>>>>>   Userspace APIC is broken since it may run with an outdated cr8, 
> >>>>>>>> does
> >>>>>>>>   reverting 27a4f7976d5 help?
> >>>>>>>
> >>>>>>>  Can you elaborate on what is broken? The way hw/apic.c maintains the
> >>>>>>>  tpr? Would it make sense to compare this against the in-kernel 
> >>>>>>> model? Or
> >>>>>>>  do you mean something else?
> >>>>>>
> >>>>>>  The problem, IIRC, was that we look up the TPR but it may already have
> >>>>>>  been changed by the running vcpu.  Not 100% sure.
> >>>>>>
> >>>>>>  If that is indeed the problem then the fix would be to process the 
> >>>>>> APIC
> >>>>>>  in vcpu context (which is what the kernel does - we set a bit in the 
> >>>>>> IRR
> >>>>>>  and all further processing is synchronous).
> >>>>>
> >>>>>  You mean: user space changes the tpr value while the vcpu is in 
> >>>>> KVM_RUN,
> >>>>>  then we return from the kernel and overwrite the tpr in the apic with
> >>>>>  the vcpu's view, right?
> >>>>
> >>>> Hmm, probably rather that there is a discrepancy between tpr and irr.
> >>>> The latter is changed asynchronously /wrt to the vcpu, the former /wrt
> >>>> the user space device model.
> >>>
> >>> And yet, both are synchronized via qemu_mutex.  So we're still missing 
> >>> something in this picture.
> >>>
> >>>> Run apic_set_irq on the vcpu?
> >>>
> >>> static void apic_set_irq(APICState *s, int vector_num, int trigger_mode)
> >>> {
> >>>      apic_irq_delivered += !get_bit(s->irr, vector_num);
> >>>
> >>>      trace_apic_set_irq(apic_irq_delivered);
> >>>
> >>>      set_bit(s->irr, vector_num);
> >>>
> >>> This is even more async with kernel irqchip
> >>>
> >>>      if (trigger_mode)
> >>>          set_bit(s->tmr, vector_num);
> >>>      else
> >>>          reset_bit(s->tmr, vector_num);
> >>>
> >>> This is protected by qemu_mutex
> >>>
> >>>      apic_update_irq(s);
> >>>
> >>> This will be run the next time the vcpu exits, via apic_get_interrupt().
> >>
> >> The decision to pend an IRQ (and potentially kick the vcpu) takes place
> >> immediately in acip_update_irq. And it is based on current irr as well
> >> as tpr. But we update again when user space returns with a new value.
> >>
> >>>
> >>> }
> >>>
> >>> Did you check whether reverting that commit helps?
> >>>
> >>
> >> Just did so, and I can no longer reproduce the problem. Hmm...
> >>
> > If there is no problem in the logic of this commit (and I do not see
> > one yet) then we somewhere miss kicking vcpu when interrupt, that should be
> > handled, arrives?
> 
> I'm not yet confident about the logic of the kernel patch: mov to cr8 is
> serializing. If the guest raises the tpr and then signals this with a
> succeeding, non vm-exiting instruction to the other vcpus, one of those
> could inject an interrupt with a higher priority than the previous tpr,
> but a lower one than current tpr. QEMU user space would accept this
> interrupt - and would likely surprise the guest. Do I miss something?
> 
Injection happens by vcpu thread on cpu entry:
run->request_interrupt_window = kvm_arch_try_push_interrupts(env);
and tpr is synced on vcpu exit, so I do not yet see how what you describe
above may happen since during injection vcpu should see correct tpr.

--
                        Gleb.