> On 19 Mar 2019, at 23:06, Michael S. Tsirkin <m...@redhat.com> wrote:
> On Tue, Mar 19, 2019 at 02:38:06PM +0200, Liran Alon wrote:
>> Hi Michael,
>> Great blog-post which summarise everything very well!
>> Some comments I have:
> Thanks!
> I'll try to update everything in the post when I'm not so jet-lagged.
>> 1) I think that when we are using the term “1-netdev model” on community 
>> discussion, we tend to refer to what you have defined in blog-post as 
>> "3-device model with hidden slaves”.
>> Therefore, I would suggest to just remove the “1-netdev model” section and 
>> rename the "3-device model with hidden slaves” section to “1-netdev model”.
>> 2) The userspace issues result both from using “2-netdev model” and 
>> “3-netdev model”. However, they are described in blog-post as they only 
>> exist on “3-netdev model”.
>> The reason these issues are not seen in Azure environment is because these 
>> issues were partially handled by Microsoft for their specific 2-netdev model.
>> Which leads me to the next comment.
>> 3) I suggest that blog-post will also elaborate on what exactly are the 
>> userspace issues which results in models different than “1-netdev model”.
>> The issues that I’m aware of are (Please tell me if you are aware of 
>> others!):
>> (a) udev rename race-condition: When net-failover device is opened, it also 
>> opens it's slaves. However, the order of events to udev on KOBJ_ADD is first 
>> for the net-failover netdev and only then for the virtio-net netdev. This 
>> means that if userspace will respond to first event by open the 
>> net-failover, then any attempt of userspace to rename virtio-net netdev as a 
>> response to the second event will fail because the virtio-net netdev is 
>> already opened. Also note that this udev rename rule is useful because we 
>> would like to add rules that renames virtio-net netdev to clearly signal 
>> that it’s used as the standby interface of another net-failover netdev.
>> The way this problem was workaround by Microsoft in NetVSC is to delay the 
>> open done on slave-VF from the open of the NetVSC netdev. However, this is 
>> still a race and thus a hacky solution. It was accepted by community only 
>> because it’s internal to the NetVSC driver. However, similar solution was 
>> rejected by community for the net-failover driver.
>> The solution that we currently proposed to address this (Patch by Si-Wei) 
>> was to change the rename kernel handling to allow a net-failover slave to be 
>> renamed even if it is already opened. Patch is still not accepted.
>> (b) Issues caused because of various userspace components DHCP the 
>> net-failover slaves: DHCP of course should only be done on the net-failover 
>> netdev. Attempting to DHCP on net-failover slaves as-well will cause 
>> networking issues. Therefore, userspace components should be taught to avoid 
>> doing DHCP on the net-failover slaves. The various userspace components 
>> include:
>> b.1) dhclient: If run without parameters, it by default just enum all 
>> netdevs and attempt to DHCP them all.
>> (I don’t think Microsoft has handled this)
>> b.2) initramfs / dracut: In order to mount the root file-system from iSCSI, 
>> these components needs networking and therefore DHCP on all netdevs.
>> (Microsoft haven’t handled (b.2) because they don’t have images which 
>> perform iSCSI boot in their Azure setup. Still an open issue)
>> b.3) cloud-init: If configured to perform network-configuration, it attempts 
>> to configure all available netdevs. It should avoid however doing so on 
>> net-failover slaves.
>> (Microsoft has handled this by adding a mechanism in cloud-init to blacklist 
>> a netdev from being configured in case it is owned by a specific PCI driver. 
>> Specifically, they blacklist Mellanox VF driver. However, this technique 
>> doesn’t work for the net-failover mechanism because both the net-failover 
>> netdev and the virtio-net netdev are owned by the virtio-net PCI driver).
>> b.4) Various distros network-manager need to be updated to avoid DHCP on 
>> net-failover slaves? (Not sure. Asking...)
>> 4) Another interesting use-case where the net-failover mechanism is useful 
>> is for handling NIC firmware failures or NIC firmware Live-Upgrade.
>> In both cases, there is a need to perform a full PCIe reset of the NIC. 
>> Which lose all the NIC eSwitch configuration of the various VFs.
> In this setup, how does VF keep going? If it doesn't keep going, why is
> it helpful?

Let me attempt to clarify.

First, let’s analyse what can a cloud provider do when it wishes to upgrade the 
NIC firmware when there are currently running guests utilising SR-IOV.
He can perform the following operations in order:
1) Hot-unplug all VFs from all running guests.
2) Upgrade NIC firmware. Will result in PCIe reset which will cause momentary 
network down-time on PF but immediately afterwards PF will be set up again and 
guests will have network connectivity.
3) Provision and hot-plug new VFs for all running guests. Guests again have 
accelerated networking.

Without the net-failover mechanism, host will have to hot-unplug all VFs from 
all running guests and provision new VFs and hot-plug them anyway. But in that 
case, the network down-time for guests is longer.

Second, let’s analyse what will happen when health service running on host 
notice that NIC firmware is in a bad state and therefore NIC should be reset to 
The health service can take exactly the same order of operations as described 
above besides (2) which will just become a PCIe reset.
Again, guests have shorter network down-time in this case as-well when 
utilising the net-failover mechanism.

>> To handle these cases gracefully, one could just hot-unplug all VFs from 
>> guests running on host (which will make all guests now use the virtio-net 
>> netdev which is backed by a netdev that eventually is on top of PF). 
>> Therefore, networking will be restored to guests once the PCIe reset is 
>> completed and the PF is functional again. To re-acceelrate the guests 
>> network, hypervisor can just hot-plug new VFs to guests.
>> P.S:
>> I would very appreciate all this forum help in closing on the pending items 
>> written in (3). Which currently prevents using this net-failover mechanism 
>> in real production use-cases.
>> Regards,
>> -Liran
>>> On 17 Mar 2019, at 15:55, Michael S. Tsirkin <m...@redhat.com> wrote:
>>> Hi all,
>>> I've put up a blog post with a summary of where network
>>> device failover stands and some open issues.
>>> Not sure where best to host it, I just put it up on blogspot:
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__mstsirkin.blogspot.com_2019_03_virtio-2Dnetwork-2Ddevice-2Dfailover-2Dsupport.html&d=DwIBAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Jk6Q8nNzkQ6LJ6g42qARkg6ryIDGQr-yKXPNGZbpTx0&m=jd0emHx6EkPSTvO0TytfYmG4rOMQ9htenhrgKprrh9E&s=5EJamlc_g1lZa0Ga7K30E6aWVg3jy8lizhw1aSguo3A&e=
>>> Comments, corrections are welcome!
>>> -- 
>>> MST

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