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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-02-03 11:04, Marcelo Tosatti wrote:
> On Thu, Feb 03, 2011 at 10:32:25AM +0100, Jan Kiszka wrote:
>> On 2011-02-03 09:18, Avi Kivity wrote:
>>> On 02/02/2011 05:52 PM, Jan Kiszka wrote:
>>>>>>
>>>>>  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?
>>>
>>> apic_get_interrupt() is only called from the vcpu thread, so it should 
>>> see a correct tpr.
>>>
>>> The only difference I can see with the patch is that we may issue a 
>>> spurious cpu_interrupt().  But that shouldn't do anything bad, should it?
>>
>> I tested this yesterday, and it doesn't confuse Windows. It actually
>> receives quite a few spurious IRQs in normal operation, w/ or w/o the
>> kernel's tpr optimization.
> 
> http://www.mail-archive.com/kvm@vger.kernel.org/msg41681.html

Don't get the scenario yet: We do not inject (or set isr) over the
context of apic_set_irq caller.

> 
> tpr of a vcpu should always be inspected in vcpu context, instead of 
> iothread context?

Maybe this is true for the in-kernel model, but I don't see the issue
(anymore) for the way user space works.

Jan

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