Re: [dpdk-dev] [PATCH v5 resend 07/12] virtio: resolve for control queue

[Nikita Kalyazin <n.kalyazin@samsung.com>]

Hi Yuanhan,


As I understand, the dead loop happened here (virtio_send_command):
while (vq->vq_used_cons_idx == vq->vq_ring.used->idx) {
      rte_rmb();
      usleep(100);
}

Could you explain why wrong config reading caused that and how correct reading helps to avoid?

-- 

Best regards,

Nikita Kalyazin,
n.kalyazin@samsung.com

Software Engineer
Virtualization Group
Samsung R&D Institute Russia
Tel: +7 (495) 797-25-00 #3816
Tel: +7 (495) 797-25-03
Office #1501, 12-1, Dvintsev str.,
Moscow, 127018, Russia

On Mon, Sep 21, 2015 at 02:36:47PM +0800, Yuanhan Liu wrote:
> On Sun, Sep 20, 2015 at 12:21:14PM +0300, Michael S. Tsirkin wrote:
> > On Fri, Sep 18, 2015 at 11:10:56PM +0800, Yuanhan Liu wrote:
> > > From: Changchun Ouyang <changchun.ouyang@intel.com>
> > > 
> > > Fix the max virtio queue pair read issue.
> > > 
> > > Control queue can't work for vhost-user mulitple queue mode,
> > > so introduce a counter to void the dead loop when polling
> > > the control queue.
> > > 
> > > Signed-off-by: Changchun Ouyang <changchun.ouyang@intel.com>
> > > Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
> > 
> > Per virtio spec, the multiqueue feature depends on control queue -
> > what do you mean when you say it can't work?
> > 
> > > ---
> > >  drivers/net/virtio/virtio_ethdev.c | 12 +++++++-----
> > >  1 file changed, 7 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/drivers/net/virtio/virtio_ethdev.c b/drivers/net/virtio/virtio_ethdev.c
> > > index 465d3cd..b2f4120 100644
> > > --- a/drivers/net/virtio/virtio_ethdev.c
> > > +++ b/drivers/net/virtio/virtio_ethdev.c
> > > @@ -1162,7 +1162,6 @@ eth_virtio_dev_init(struct rte_eth_dev *eth_dev)
> > >  	struct virtio_hw *hw = eth_dev->data->dev_private;
> > >  	struct virtio_net_config *config;
> > >  	struct virtio_net_config local_config;
> > > -	uint32_t offset_conf = sizeof(config->mac);
> > >  	struct rte_pci_device *pci_dev;
> > >  
> > >  	RTE_BUILD_BUG_ON(RTE_PKTMBUF_HEADROOM < sizeof(struct virtio_net_hdr));
> > > @@ -1222,7 +1221,9 @@ eth_virtio_dev_init(struct rte_eth_dev *eth_dev)
> > >  		config = &local_config;
> > >  
> > >  		if (vtpci_with_feature(hw, VIRTIO_NET_F_STATUS)) {
> > > -			offset_conf += sizeof(config->status);
> > > +			vtpci_read_dev_config(hw,
> > > +				offsetof(struct virtio_net_config, status),
> > > +				&config->status, sizeof(config->status));
> > >  		} else {
> > >  			PMD_INIT_LOG(DEBUG,
> > >  				     "VIRTIO_NET_F_STATUS is not supported");
> > > @@ -1230,15 +1231,16 @@ eth_virtio_dev_init(struct rte_eth_dev *eth_dev)
> > >  		}
> > >  
> > >  		if (vtpci_with_feature(hw, VIRTIO_NET_F_MQ)) {
> > > -			offset_conf += sizeof(config->max_virtqueue_pairs);
> > > +			vtpci_read_dev_config(hw,
> > > +				offsetof(struct virtio_net_config, max_virtqueue_pairs),
> > > +				&config->max_virtqueue_pairs,
> > > +				sizeof(config->max_virtqueue_pairs));
> > >  		} else {
> > >  			PMD_INIT_LOG(DEBUG,
> > >  				     "VIRTIO_NET_F_MQ is not supported");
> > >  			config->max_virtqueue_pairs = 1;
> > >  		}
> > >  
> > > -		vtpci_read_dev_config(hw, 0, (uint8_t *)config, offset_conf);
> > > -
> > >  		hw->max_rx_queues =
> > >  			(VIRTIO_MAX_RX_QUEUES < config->max_virtqueue_pairs) ?
> > >  			VIRTIO_MAX_RX_QUEUES : config->max_virtqueue_pairs;
> > 
> > 
> > Does the patch actually do what the commit log says?
> 
> Sorry, the commit log is wrong as you said.
> 
> It was actually a bug in our code, which happens to be revealed when
> MQ is enabled. The old code adjusts the config bytes we want to read
> depending on what kind of features we have, but we later cast the
> entire buf we read with "struct virtio_net_config", which is obviously
> wrong.
> 
> The right way to go is to read related config bytes when corresponding
> feature is set, which is exactly what this patch does.
> 
> > It seems tobe about reading the device confing,
> > not breaking out of a loop ...
> 
> It's just a (bad) side effect of getting the vritio_net_config wrongly:
> the wrong config causes a dead loop in our code.
> 
> And sorry for the buggy commit log, will fix it next version.
> 
> Thanks.
> 
> 	--yliu