Re: [Qemu-devel] Symptoms of lost interrupts in virtio?

[Michael S. Tsirkin]

On Mon, Sep 03, 2012 at 03:16:00PM +0200, Soren Hansen wrote:
> Michael,
> 
> In Ubuntu we're seeing a bug in qemu-kvm 1.0 where networking stops
> working. It's described here:
> 
>    https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/997978
> 
> We've not been able to reproduce it ourselves, but the users who are
> experiencing it say that it's fixed in 1.1.
> 
> I've found this series of commits:
> 
>    a821ce5 virtio: order index/descriptor reads
>    92045d8 virtio: add missing mb() on enable notification
>    a281ebc virtio: add missing mb() on notification
> 
> where particularly the last one looks like a likely fix for the problem.
> 
> The commit message isn't entirely clear about the symptoms it's meant
> to fix, even though the behind-the-covers explanation is very
> thorough. It's not clear to me, for instance, if the network would
> stop working or if you'd just lose a couple of network packets?


It can stop working. It's more likely to stop if
event_idx is enabled. Try disabling and see what happens.



> The commit message suggests it only happens on a very specific
> processor type. Do you happen to remember which one? If it matches
> what our affected users see, that could be clue as to whether this is
> the  same problem.

I think it was reported on AMD systems.


> Thanks in advance for your time.
> 
> -- 
> Soren Hansen
> Ubuntu Developer    http://www.ubuntu.com/
> OpenStack Developer http://www.openstack.org/