Re: [PATCH v4 2/2] virtio_net: Extend virtio to use VF datapath when available

Siwei Liu
2018-03-14T04:50:04Z

On Tue, Mar 13, 2018 at 5:28 PM, Samudrala, Sridhar
<sridhar.samudrala@intel.com> wrote:
> On 3/12/2018 3:44 PM, Siwei Liu wrote:
>>
>> Apologies, still some comments going. Please see inline.
>>
>> On Thu, Mar 1, 2018 at 12:08 PM, Sridhar Samudrala
>> <sridhar.samudrala@intel.com> wrote:
>>>
>>> This patch enables virtio_net to switch over to a VF datapath when a VF
>>> netdev is present with the same MAC address. It allows live migration
>>> of a VM with a direct attached VF without the need to setup a bond/team
>>> between a VF and virtio net device in the guest.
>>>
>>> The hypervisor needs to enable only one datapath at any time so that
>>> packets don't get looped back to the VM over the other datapath. When a
>>> VF
>>> is plugged, the virtio datapath link state can be marked as down. The
>>> hypervisor needs to unplug the VF device from the guest on the source
>>> host
>>> and reset the MAC filter of the VF to initiate failover of datapath to
>>> virtio before starting the migration. After the migration is completed,
>>> the destination hypervisor sets the MAC filter on the VF and plugs it
>>> back
>>> to the guest to switch over to VF datapath.
>>>
>>> When BACKUP feature is enabled, an additional netdev(bypass netdev) is
>>> created that acts as a master device and tracks the state of the 2 lower
>>> netdevs. The original virtio_net netdev is marked as 'backup' netdev and
>>> a
>>> passthru device with the same MAC is registered as 'active' netdev.
>>>
>>> This patch is based on the discussion initiated by Jesse on this thread.
>>> https://marc.info/?l=linux-virtualization&m=151189725224231&w=2
>>>
>>> Signed-off-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
>>> Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
>>> ---
>>> drivers/net/virtio_net.c | 683
>>> ++++++++++++++++++++++++++++++++++++++++++++++-
>>> 1 file changed, 682 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index bcd13fe906ca..f2860d86c952 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -30,6 +30,8 @@
>>> #include <linux/cpu.h>
>>> #include <linux/average.h>
>>> #include <linux/filter.h>
>>> +#include <linux/netdevice.h>
>>> +#include <linux/pci.h>
>>> #include <net/route.h>
>>> #include <net/xdp.h>
>>>
>>> @@ -206,6 +208,9 @@ struct virtnet_info {
>>> u32 speed;
>>>
>>> unsigned long guest_offloads;
>>> +
>>> + /* upper netdev created when BACKUP feature enabled */
>>> + struct net_device *bypass_netdev;
>>> };
>>>
>>> struct padded_vnet_hdr {
>>> @@ -2236,6 +2241,22 @@ static int virtnet_xdp(struct net_device *dev,
>>> struct netdev_bpf *xdp)
>>> }
>>> }
>>>
>>> +static int virtnet_get_phys_port_name(struct net_device *dev, char *buf,
>>> + size_t len)
>>> +{
>>> + struct virtnet_info *vi = netdev_priv(dev);
>>> + int ret;
>>> +
>>> + if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_BACKUP))
>>> + return -EOPNOTSUPP;
>>> +
>>> + ret = snprintf(buf, len, "_bkup");
>>> + if (ret >= len)
>>> + return -EOPNOTSUPP;
>>> +
>>> + return 0;
>>> +}
>>> +
>>> static const struct net_device_ops virtnet_netdev = {
>>> .ndo_open = virtnet_open,
>>> .ndo_stop = virtnet_close,
>>> @@ -2253,6 +2274,7 @@ static const struct net_device_ops virtnet_netdev =
>>> {
>>> .ndo_xdp_xmit = virtnet_xdp_xmit,
>>> .ndo_xdp_flush = virtnet_xdp_flush,
>>> .ndo_features_check = passthru_features_check,
>>> + .ndo_get_phys_port_name = virtnet_get_phys_port_name,
>>> };
>>>
>>> static void virtnet_config_changed_work(struct work_struct *work)
>>> @@ -2647,6 +2669,653 @@ static int virtnet_validate(struct virtio_device
>>> *vdev)
>>> return 0;
>>> }
>>>
>>> +/* START of functions supporting VIRTIO_NET_F_BACKUP feature.
>>> + * When BACKUP feature is enabled, an additional netdev(bypass netdev)
>>> + * is created that acts as a master device and tracks the state of the
>>> + * 2 lower netdevs. The original virtio_net netdev is registered as
>>> + * 'backup' netdev and a passthru device with the same MAC is registered
>>> + * as 'active' netdev.
>>> + */
>>> +
>>> +/* bypass state maintained when BACKUP feature is enabled */
>>> +struct virtnet_bypass_info {
>>> + /* passthru netdev with same MAC */
>>> + struct net_device __rcu *active_netdev;
>>> +
>>> + /* virtio_net netdev */
>>> + struct net_device __rcu *backup_netdev;
>>> +
>>> + /* active netdev stats */
>>> + struct rtnl_link_stats64 active_stats;
>>> +
>>> + /* backup netdev stats */
>>> + struct rtnl_link_stats64 backup_stats;
>>> +
>>> + /* aggregated stats */
>>> + struct rtnl_link_stats64 bypass_stats;
>>> +
>>> + /* spinlock while updating stats */
>>> + spinlock_t stats_lock;
>>> +};
>>> +
>>> +static void virtnet_bypass_child_open(struct net_device *dev,
>>> + struct net_device *child_netdev)
>>> +{
>>> + int err = dev_open(child_netdev);
>>> +
>>> + if (err)
>>> + netdev_warn(dev, "unable to open slave: %s: %d\n",
>>> + child_netdev->name, err);
>>> +}
>>
>> Why ignoring the error?? i.e. virtnet_bypass_child_open should have
>> return value. I don't believe the caller is in a safe context to
>> guarantee the dev_open always succeeds.
>
>
> OK. Will handle this in the next revision.
>
>
>>
>>> +
>>> +static int virtnet_bypass_open(struct net_device *dev)
>>> +{
>>> + struct virtnet_bypass_info *vbi = netdev_priv(dev);
>>> + struct net_device *child_netdev;
>>> +
>>> + netif_carrier_off(dev);
>>> + netif_tx_wake_all_queues(dev);
>>> +
>>> + child_netdev = rtnl_dereference(vbi->active_netdev);
>>> + if (child_netdev)
>>> + virtnet_bypass_child_open(dev, child_netdev);
>>
>> Handle the error?
>>
>>> +
>>> + child_netdev = rtnl_dereference(vbi->backup_netdev);
>>> + if (child_netdev)
>>> + virtnet_bypass_child_open(dev, child_netdev);
>>
>> Handle the error and unwind?
>
>
> Sure.
>
> <snip>
>
>
>> +
>> +static int virtnet_bypass_register_child(struct net_device *child_netdev)
>> +{
>> + struct virtnet_bypass_info *vbi;
>> + struct net_device *dev;
>> + bool backup;
>> + int ret;
>> +
>> + if (child_netdev->addr_len != ETH_ALEN)
>> + return NOTIFY_DONE;
>> +
>> + /* We will use the MAC address to locate the virtnet_bypass netdev
>> + * to associate with the child netdev. If we don't find a matching
>> + * bypass netdev, move on.
>> + */
>> + dev = get_virtnet_bypass_bymac(dev_net(child_netdev),
>> + child_netdev->perm_addr);
>> + if (!dev)
>> + return NOTIFY_DONE;
>> +
>> + vbi = netdev_priv(dev);
>> + backup = (child_netdev->dev.parent == dev->dev.parent);
>> + if (backup ? rtnl_dereference(vbi->backup_netdev) :
>> + rtnl_dereference(vbi->active_netdev)) {
>> + netdev_info(dev,
>> + "%s attempting to join bypass dev when %s
>> already present\n",
>> + child_netdev->name, backup ? "backup" :
>> "active");
>> + return NOTIFY_DONE;
>> + }
>> +
>> + /* Avoid non pci devices as active netdev */
>> + if (!backup && (!child_netdev->dev.parent ||
>> + !dev_is_pci(child_netdev->dev.parent)))
>> + return NOTIFY_DONE;
>> +
>> There's a problem here in terms of error (particularly on the active
>> slave, e.g. VF), see below:
>>
>>> + ret = netdev_rx_handler_register(child_netdev,
>>> + virtnet_bypass_handle_frame,
>>> dev);
>>> + if (ret != 0) {
>>> + netdev_err(child_netdev,
>>> + "can not register bypass receive handler (err
>>> = %d)\n",
>>> + ret);
>>> + goto rx_handler_failed;
>>> + }
>>> +
>>> + ret = netdev_upper_dev_link(child_netdev, dev, NULL);
>>> + if (ret != 0) {
>>> + netdev_err(child_netdev,
>>> + "can not set master device %s (err = %d)\n",
>>> + dev->name, ret);
>>> + goto upper_link_failed;
>>> + }
>>> +
>>> + child_netdev->flags |= IFF_SLAVE;
>>> +
>>> + if (netif_running(dev)) {
>>> + ret = dev_open(child_netdev);
>>> + if (ret && (ret != -EBUSY)) {
>>> + netdev_err(dev, "Opening child %s failed
>>> ret:%d\n",
>>> + child_netdev->name, ret);
>>> + goto err_interface_up;
>>> + }
>>> + }
>>> +
>>> + /* Align MTU of child with master */
>>> + ret = dev_set_mtu(child_netdev, dev->mtu);
>>> + if (ret) {
>>> + netdev_err(dev,
>>> + "unable to change mtu of %s to %u register
>>> failed\n",
>>> + child_netdev->name, dev->mtu);
>>> + goto err_set_mtu;
>>> + }
>>
>> If any of the above calls returns non-zero, the current code steps
>> back to undo what's being done on that spefic slave previously. For
>> instance, if the netdev_rx_handler_register returns non-zero because
>> of busy rx handler, this register_child function would give up
>> enslaving the VF and leave the upper virtio_bypass interface behind
>> once it returns.
>
>
> virtio_bypass interface is the upper master netdev and it is always present
> when
> BACKUP feature is enabled.
> If there is failure with enslaving VF, there will be 2 netdevs,
> master virtio_bypass and slave virtio_net and the VM can be migrated.

Failure in enslaving the VF does not mean the netdev would disappear.
That means there will be 3 seperate netdevs left behind, a master
virtio_bypass with slave virtio_net, plus a standalone VF. In this
case the VF are still useable as the datapath is still there, while
the datapath for virtio_* is made standby. Which interface you expect
user to continue to use in case of failure?

If it's the VF interface you choose, there should be some detection
for that condition in virtio_net, hide the virtio interfaces from user
by unregistering those two virtio netdevs (master virtio_bypass with
slave virtio_net), presumably.

If not so user has to use virtio_bypass, how do you prevent users from
using the VF, or consequently, indicate/propogate the failure to
backend and switch the host datapath to virtio?

I don't see either of the above being handled on your patch. Unless I
miss something obvious, I don't think this patch is close to achieving
the goal in terms of completeness.

>
>
>> I am not sure if it's a good idea to leave the
>> virtio_bypass around if running into failure: the guest is not
>> migratable as the VF doesn't have a backup path,
>
>
> Are you talking about a failure when registering backup netdev? This should

No, see above. I was mainly concerned with the failure in enslaving VF
(active netdev).

If users continue to use raw VF interface within the VM, the guest
cannot be migrated for sure.

> not
> happen, but i guess we can improve error handling in such scenario.
>
>
>> And perhaps the worse
>> part is that, it now has two interfaces with identical MAC address but
>> one of them is invalid (user cannot use the virtio interface as it has
>> a dampened datapath). IMHO the virtio_bypass and its lower netdev
>> should be destroyed at all when it fails to bind the VF, and
>> technically, there should be some way to propogate the failure status
>> to the hypervisor/backend, indicating that the VM is not migratable
>> because of guest software errors (maybe by clearing out the backup
>> feature from the guest virtio driver so host can see/learn it).
>
>
> In BACKUP mode, user can only use the upper virtio_bypass netdev and that
> will
> always be there. Any failure to enslave VF netdev is not fatal, but i will
> see
> if we can improve the error handling of failure to enslave backup netdev.
> Also, i don't think the BACKUP feature bit is negotiable with the host.

I don't believe that is the case. In any event, you should come up
with whatever means to indicate to the hypervisor that some VF are
still left unbound with a virtio, so QEMU knows the VM cannot live
migrate when host user attempts to do so. Or, alternatively, propoate
the failure to host, plug out the VF and switch the host datapath to
virtio. Please work out a complete solution to address the error
case.

Regards,
-Siwei

>
> Thanks
> Sridhar
>
>