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

[Jan Kiszka]

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   Userspace APIC is broken since it may run with an outdated cr8, does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...

Jan

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