Message-Id: <A1D42CA2-1039-4F22-9128-37501E66922D@oracle.com>
Date:   Wed, 20 Mar 2019 01:05:51 +0200
From:   Liran Alon <liran.alon@...cle.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Si-Wei Liu <si-wei.liu@...cle.com>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Jakub Kicinski <kubakici@...pl>, Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        boris.ostrovsky@...cle.com, vijay.balakrishna@...cle.com,
        jfreimann@...hat.com, ogerlitz@...lanox.com, vuhuong@...lanox.com
Subject: Re: [summary] virtio network device failover writeup



> On 19 Mar 2019, at 23:06, Michael S. Tsirkin <mst@...hat.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 recover.
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 <mst@...hat.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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