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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)

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 :-)

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