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SubjectRe: [PATCH] kvm: ioapic: conditionally delay irq delivery during eoi broadcast
On 03/12/2012 05:23 PM, Gleb Natapov wrote:
> On Mon, Mar 12, 2012 at 05:07:35PM +0800, Jason Wang wrote:
>> > Currently, we call ioapic_service() immediately when we find the irq is still
>> > active during eoi broadcast. But for real hardware, there's some dealy between
>> > the EOI writing and irq delivery (system bus latency?). So we need to emulate
>> > this behavior. Otherwise, for a guest who haven't register a proper irq handler
>> > , it would stay in the interrupt routine as this irq would be re-injected
>> > immediately after guest enables interrupt. This would lead guest can't move
>> > forward and may miss the possibility to get proper irq handler registered (one
>> > example is windows guest resuming from hibernation).
>> >
> Yes, I saw this behaviour with Windows NICs, but it looks like the
> guest bug. Does this happen with other kind of devices too? Because
> if it does not then the correct hack would be to add a delay between
> Windows enabling PHY and sending first interrupt to a guest. This will
> model what happens on real HW. NIC does not start receiving packets at
> the same moment PHY is enabled. Some time is spent bring up the link.
>

Looks common for any unhandled level irq but I haven't tried. What I've
tested is running a similar test program by hacking the card driver and
let it run in both real physical machine and a kvm guest, and see what
happens if there's no irq handled:

- In real hardware, there's a gap between two successive irqs injected
by eoi broadcast, and OS can move forward.
- In a kvm guest, no gap, guest can't move forward and would always stay
in the irq context forever.


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