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Re: [Xen-devel] [VTD][patch 0/5] HVM device assignment using vt-d

To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, Guy Zana <guy@xxxxxxxxxxxx>, "Kay, Allen M" <allen.m.kay@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [VTD][patch 0/5] HVM device assignment using vt-d
From: Keir Fraser <keir@xxxxxxxxxxxxx>
Date: Thu, 31 May 2007 14:37:18 +0100
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On 31/5/07 14:20, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

>     When driver requests device to clear interrupt assertion at end
> of handling 1st, it's possible that device keeps assertion if interrupt
> condition still matches in 2nd. In that case, no interrupt will happen
> any more when EOI is written to IOAPIC due to polarity inversion.

This is absolutely fine. The virtual wire status will remain HIGH in this
case, which is correct since the 'runt' LOW pulse on the physical wire can
be ignored. What we are looking for is to track the physical wire status in
the long run; pulses one way or the other do not matter.

Remember we are talking about *level-triggered* interrupt lines, not
edge-triggered. This polarity-change trick would not be used, and would not
be necessary, for edge-triggered interrupts. We would EOI the physical APIC
early, before running the ISR, just as usual for edge-triggered interrupts.

Because we are talking about level-triggered interrupts, if the line
continues to be HIGH after the ISR runs then of course we'll just deliver
another interrupt straight to the relevant VCPU. That's how level-triggered
interrupts work. :-)

 -- Keir

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