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

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Date:   Mon, 16 Oct 2017 18:34:30 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jason Wang <jasowang@...hat.com>,
        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 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.

>
>> >> > Also, it seems that LINK_ANNOUNCE requests will get ignored
>> >> > even if they got set before the reset, leading to downtime.
>> >>
>> >> Do you mean act on VIRTIO_NET_F_GUEST_ANNOUNCE
>> >> requests? That flag is tested and netdev_notify_peers
>> >> called before resetting virtio ring state.
>> >
>> > Yes but I wonder if there's a race where announce
>> > is set after it is read but before NEED_RESET is read.
>> >
>> > Re-reading status from the config before reset
>> > might be necessary.
>>
>> Thanks, I'll have a look. Perhaps a host should simply not
>> request a reset while it is waiting for an announce ack.
>
> It's one option though we can't make this change for existing hosts.
> We also have the reverse condition where announce is requested after
> NEED_RESET is set.