Re: [PATCH 1/2] kvm tools: Respect ISR status in virtio header

[Ingo Molnar <mingo@elte.hu>]

* Pekka Enberg <penberg@kernel.org> wrote:

> On Sat, May 7, 2011 at 6:01 PM, Anthony Liguori <aliguori@us.ibm.com> wrote:
> > On 05/07/2011 09:50 AM, Pekka Enberg wrote:
> >>
> >> On Sat, 2011-05-07 at 08:14 -0500, Anthony Liguori wrote:
> >>>
> >>> It seems like this patch is simply avoiding raising the interrupt line
> >>> if the ISR has not been acknowledged yet.  I don't think there's a
> >>> functional issue here but I'm surprised that it's a win.  There should
> >>> be a very short window when the interrupt is lowered in the APIC but
> >>> still not acknowledged in the ISR.
> >>>
> >>> You should just be saving a pretty cheap system call.  I wonder if the
> >>> system call is taking longer than it should..
> >>
> >> The patch seems to fix a bug where the guest kernel breaks down under
> >> interrupt storm and stops doing VIRTIO_PCI_QUEUE_NOTIFY. We assumed it
> >> was something with our code but your comments make me wonder if there's
> >> a real problem in KVM_IRQ_LINE.
> >
> > Stops doing it for a short period of time or entirely?
> 
> Seems to be entirely. The test case is doing "ping -f" from host to
> guest and vice versa and it takes 30-60 seconds to trigger for me.

here's the condition as described by Asias:

" I also found when network hangs, guest kernel refuse to give any avail 
  buffers in rx queue to device. At that time, vq->last_used_idx equals 
  vq->vring.used->idx in rx queue, so even with manual IRQ injection using a 
  debug key ctrl-a-i, the network still hangs. "

Thanks,

	Ingo