netdev - Re: [PATCH net-next] virtio_net: implement VIRTIO_CONFIG_S_NEEDS

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Date:   Thu, 28 Dec 2017 14:11:12 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        virtualization@...ts.linux-foundation.org,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] virtio_net: implement VIRTIO_CONFIG_S_NEEDS_RESET

On Mon, Oct 16, 2017 at 11:44 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Tue, Oct 17, 2017 at 11:05:07AM +0800, Jason Wang wrote:
>>
>>
>> On 2017年10月17日 06:34, Willem de Bruijn wrote:
>> > On Mon, Oct 16, 2017 at 12:38 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
>> > > On Mon, Oct 16, 2017 at 12:04:57PM -0400, Willem de Bruijn wrote:
>> > > > On Mon, Oct 16, 2017 at 11:31 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
>> > > > > On Mon, Oct 16, 2017 at 11:03:18AM -0400, Willem de Bruijn wrote:
>> > > > > > > > +static int virtnet_reset(struct virtnet_info *vi)
>> > > > > > > > +{
>> > > > > > > > +     struct virtio_device *dev = vi->vdev;
>> > > > > > > > +     int ret;
>> > > > > > > > +
>> > > > > > > > +     virtio_config_disable(dev);
>> > > > > > > > +     dev->failed = dev->config->get_status(dev) & VIRTIO_CONFIG_S_FAILED;
>> > > > > > > > +     virtnet_freeze_down(dev, true);
>> > > > > > > > +     remove_vq_common(vi);
>> > > > > > > > +
>> > > > > > > > +     virtio_add_status(dev, VIRTIO_CONFIG_S_ACKNOWLEDGE);
>> > > > > > > > +     virtio_add_status(dev, VIRTIO_CONFIG_S_DRIVER);
>> > > > > > > > +
>> > > > > > > > +     ret = virtio_finalize_features(dev);
>> > > > > > > > +     if (ret)
>> > > > > > > > +             goto err;
>> > > > > > > > +
>> > > > > > > > +     ret = virtnet_restore_up(dev);
>> > > > > > > > +     if (ret)
>> > > > > > > > +             goto err;
>> > > > > > > > +
>> > > > > > > > +     ret = virtnet_set_queues(vi, vi->curr_queue_pairs);
>> > > > > > > > +     if (ret)
>> > > > > > > > +             goto err;
>> > > > > > > > +
>> > > > > > > > +     virtio_add_status(dev, VIRTIO_CONFIG_S_DRIVER_OK);
>> > > > > > > > +     virtio_config_enable(dev);
>> > > > > > > > +     return 0;
>> > > > > > > > +
>> > > > > > > > +err:
>> > > > > > > > +     virtio_add_status(dev, VIRTIO_CONFIG_S_FAILED);
>> > > > > > > > +     return ret;
>> > > > > > > > +}
>> > > > > > > > +
>> > > > > > > >   static int virtnet_set_guest_offloads(struct virtnet_info *vi, u64 offloads)
>> > > > > > > >   {
>> > > > > > > >        struct scatterlist sg;
>> > > > > > > I have a question here though. How do things like MAC address
>> > > > > > > get restored?
>> > > > > > >
>> > > > > > > What about the rx mode?
>> > > > > > >
>> > > > > > > vlans?
>> > > > > > The function as is releases and reinitializes only ring state.
>> > > > > > Device configuration such as mac and vlan persist across
>> > > > > > the reset.
>> > > > > What gave you this impression? Take a look at e.g. this
>> > > > > code in qemu:
>> > > > >
>> > > > > static void virtio_net_reset(VirtIODevice *vdev)
>> > > > > {
>> > > > >      VirtIONet *n = VIRTIO_NET(vdev);
>> > > > >
>> > > > >      /* Reset back to compatibility mode */
>> > > > >      n->promisc = 1;
>> > > > >      n->allmulti = 0;
>> > > > >      n->alluni = 0;
>> > > > >      n->nomulti = 0;
>> > > > >      n->nouni = 0;
>> > > > >      n->nobcast = 0;
>> > > > >      /* multiqueue is disabled by default */
>> > > > >      n->curr_queues = 1;
>> > > > >      timer_del(n->announce_timer);
>> > > > >      n->announce_counter = 0;
>> > > > >      n->status &= ~VIRTIO_NET_S_ANNOUNCE;
>> > > > >
>> > > > >      /* Flush any MAC and VLAN filter table state */
>> > > > >      n->mac_table.in_use = 0;
>> > > > >      n->mac_table.first_multi = 0;
>> > > > >      n->mac_table.multi_overflow = 0;
>> > > > >      n->mac_table.uni_overflow = 0;
>> > > > >      memset(n->mac_table.macs, 0, MAC_TABLE_ENTRIES * ETH_ALEN);
>> > > > >      memcpy(&n->mac[0], &n->nic->conf->macaddr, sizeof(n->mac));
>> > > > >      qemu_format_nic_info_str(qemu_get_queue(n->nic), n->mac);
>> > > > >      memset(n->vlans, 0, MAX_VLAN >> 3);
>> > > > > }
>> > > > >
>> > > > > So device seems to lose all state, you have to re-program it.
>> > > > Oh, indeed! The guest does not reset its state, so it might
>> > > > be out of sync with the host after the operation. Was this not
>> > > > an issue when previously resetting in the context of xdp?
>> > > I suspect it was broken back then, too.
>> > Okay. I guess that in principle this is all programmable through
>> > virtnet_set_rx_mode, virtnet_vlan_rx_add_vid, etc. But it's a
>> > lot more complex than just restoring virtnet_reset. Will need to
>> > be careful about concurrency issues at the least. Similar to the
>> > ones you point out below.
>> >
>>
>> The problem has been pointed out during developing virtio-net XDP. But it
>> may not be a big issue since vhost_net ignores all kinds of the filters now.
>>
>> Thanks
>
> It might not keep doing that in the future though.
> And virtio-net in userspace doesn't ignore the filters.

How about the guest honor the request only if no state has been
offloaded to the host?

This is the common case for vhost_net, and not expected to change
soon.

Even when it does, we have a graceful degradation strategy. Guest
revert state prior to reset and reapply. Though for the time being,
solving this only in the case without state offload would be solve my
use case.