Re: [Qemu-devel] [PATCH v5 2/4] virtio-pci: Use ioeventfd for virtqueue notify

[Kevin Wolf <kwolf@redhat.com>]

Am 24.01.2011 20:36, schrieb Michael S. Tsirkin:
> 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?

Yes, with ioeventfd=off it seems to work fine.

Kevin