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

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Message-ID: <20120312102225.GW17882@redhat.com>
Date:	Mon, 12 Mar 2012 12:22:25 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Jason Wang <jasowang@...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 Mon, Mar 12, 2012 at 05:44:00PM +0800, Jason Wang wrote:
> 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.
This is not something an OS should rely on. So lets do the Windows
hack in QEMU NIC devices.

--
			Gleb.
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