linux-kernel - Re: [PATCH] kvm: ioapic: conditionally delay irq delivery during eoi broadcast

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Date:	Mon, 12 Mar 2012 17:44:00 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	mtosatti@...hat.com, avi@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mst@...hat.com
Subject: Re: [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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