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Date: Mon, 24 Jan 2011 21:36:53 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v5 2/4] virtio-pci: Use ioeventfd for
	virtqueue notify
Message-ID: <20110124193653.GC29941@redhat.com>
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

On Mon, Jan 24, 2011 at 07:54:20PM +0100, Kevin Wolf wrote:
> Am 12.12.2010 16:02, schrieb Stefan Hajnoczi:
> > Virtqueue notify is currently handled synchronously in userspace virtio.  This
> > prevents the vcpu from executing guest code while hardware emulation code
> > handles the notify.
> > 
> > On systems that support KVM, the ioeventfd mechanism can be used to make
> > virtqueue notify a lightweight exit by deferring hardware emulation to the
> > iothread and allowing the VM to continue execution.  This model is similar to
> > how vhost receives virtqueue notifies.
> > 
> > The result of this change is improved performance for userspace virtio devices.
> > Virtio-blk throughput increases especially for multithreaded scenarios and
> > virtio-net transmit throughput increases substantially.
> > 
> > Some virtio devices are known to have guest drivers which expect a notify to be
> > processed synchronously and spin waiting for completion.  Only enable ioeventfd
> > for virtio-blk and virtio-net for now.
> > 
> > Care must be taken not to interfere with vhost-net, which uses host
> > notifiers.  If the set_host_notifier() API is used by a device
> > virtio-pci will disable virtio-ioeventfd and let the device deal with
> > host notifiers as it wishes.
> > 
> > After migration and on VM change state (running/paused) virtio-ioeventfd
> > will enable/disable itself.
> > 
> >  * VIRTIO_CONFIG_S_DRIVER_OK -> enable virtio-ioeventfd
> >  * !VIRTIO_CONFIG_S_DRIVER_OK -> disable virtio-ioeventfd
> >  * virtio_pci_set_host_notifier() -> disable virtio-ioeventfd
> >  * vm_change_state(running=0) -> disable virtio-ioeventfd
> >  * vm_change_state(running=1) -> enable virtio-ioeventfd
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
> 
> On current git master I'm getting hangs when running iozone on a
> virtio-blk disk. "Hang" means that it's not responsive any more and has
> 100% CPU consumption.
> 
> I bisected the problem to this patch. Any ideas?
> 
> Kevin

Does it help if you set ioeventfd=off on command line?