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RE: [Xen-devel] IRQ SMP affinity problems in domU with vcpus > 4 on HP ProLiant G6 with dual Xeon 5540 (Nehalem)

He, Qing wrote:
> On Fri, 2009-10-16 at 16:22 +0800, Zhang, Xiantao wrote:
>> He, Qing wrote:
>>> On Fri, 2009-10-16 at 15:32 +0800, Zhang, Xiantao wrote:
>>>> According to the description, the issue should be caused by lost
>>>> EOI write for the MSI interrupt and leads to permanent interrupt
>>>> mask. There should be a race between guest setting new vector and 
>>>> EOIs old vector for the interrupt.  Once guest sets new vector
>>>> before it EOIs the old vector, hypervisor can't find the pirq which
>>>> corresponds old vector(has changed
>>>> to new vector) , so also can't EOI the old vector forever in
>>>> hardware level. Since the corresponding vector in real processor
>>>> can't be EOIed, so system may lose all interrupts and result the
>>>> reported issues ultimately.
>>>> But I remembered there should be a timer to handle this case
>>>> through a forcible EOI write to the real processor after timeout,
>>>> but seems it doesn't function in the expected way.
>>> The EOI timer is supposed to deal with the irq sharing problem,
>>> since MSI doesn't share, this timer will not be started in the
>>> case of MSI.
>> That maybe a problem if so. If a malicious/buggy guest won't EOI the
>> MSI vector, so host may hang due to lack of timeout mechanism?
> Why does host hang? Only the assigned interrupt will block, and that's
> exactly what the guest wants :-)

Hypervisor shouldn't EOI the real vector until guest EOI the corresponding 
virtual vector , right ?  Not sure.:-)

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