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Date: Mon, 24 Jan 2011 19:54:20 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v5 2/4] virtio-pci: Use ioeventfd for
	virtqueue notify
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

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