[dpdk-dev] [PATCH v2 2/7] net/virtio_user: postpone DRIVER OK notification

[Tan, Jianfeng]

> -----Original Message-----
> From: Yuanhan Liu [mailto:yuanhan.liu at linux.intel.com]
> Sent: Monday, December 26, 2016 2:27 PM
> To: Tan, Jianfeng
> Cc: dev at dpdk.org; Yigit, Ferruh; Liang, Cunming
> Subject: Re: [PATCH v2 2/7] net/virtio_user: postpone DRIVER OK notification
> 
> On Fri, Dec 23, 2016 at 07:14:21AM +0000, Jianfeng Tan wrote:
> > In driver probe phase, we obtain device information; and then virtio
> > driver will initialize device and stores info, like negotiated
> > features, in vhost_user layer; finally, vhost_user gets DRIVER_OK
> > notification from virtio driver, and then sync with backend device.
> >
> > Previously, DRIVER_OK could be sent twice: 1. when ether layer invokes
> > eth_device_init to initialize device; 2. when user configures it with
> > different configuration from that of previous.
> 
> Yes, but that's wrong. Now only 1) will be taken.

>From code in virtio_dev_configure() as below, if hw_ip_checksum or enable_lro is configured, the device will be reset and re-initialized.
        if (rxmode->hw_ip_checksum)
                req_features |= (1ULL << VIRTIO_NET_F_GUEST_CSUM);
        if (rxmode->enable_lro)
                req_features |=
                        (1ULL << VIRTIO_NET_F_GUEST_TSO4) |
                        (1ULL << VIRTIO_NET_F_GUEST_TSO6);

        /* if request features changed, reinit the device */
        if (req_features != hw->req_guest_features) {
                ret = virtio_init_device(dev, req_features);
                if (ret < 0)
                        return ret;
        }

> 
> > Since we can only depend on DRIVER_OK notification to sync with backend
> > device, we postpone it to virtio_dev_start when everything is settled.
> 
> I don't quite understand what you were going to fix here; you don't
> state it in the commit log after all. Normally, when everything is
> negotiated (when DRIVER_OK is set), we should not set it again, unless
> a reset has been happened.

I agree that DRIVER_OK should be set only if everything is settled. Under the case of hw_ip_checksum or enable_lro, a reset will be sent and the device will be re-initialized.

So my point is that since the configuration might be changed in dev_configure(), why not just send DRIVER_OK after everything is done.

> 
> If you look at QEMU, qemu will simply ignore it once vhost has already
> started.

It does not apply here. The configuration is changed, and it cannot be ignored.

Thanks,
Jianfeng

> 
> 	--yliu