[libvirt] [RFC 0/7] Live Migration with Pass-through Devices proposal

Chen Fan chen.fan.fnst at cn.fujitsu.com
Thu Apr 23 08:34:13 UTC 2015

On 04/20/2015 06:29 AM, Laine Stump wrote:
> On 04/17/2015 04:53 AM, Chen Fan wrote:
>> backgrond:
>> Live migration is one of the most important features of virtualization technology.
>> With regard to recent virtualization techniques, performance of network I/O is critical.
>> Current network I/O virtualization (e.g. Para-virtualized I/O, VMDq) has a significant
>> performance gap with native network I/O. Pass-through network devices have near
>> native performance, however, they have thus far prevented live migration. No existing
>> methods solve the problem of live migration with pass-through devices perfectly.
>> There was an idea to solve the problem in website:
>> https://www.kernel.org/doc/ols/2008/ols2008v2-pages-261-267.pdf
>> Please refer to above document for detailed information.
> This functionality has been on my mind/bug list for a long time, but I
> haven't been able to pursue it much. See this BZ, along with the
> original patches submitted by Shradha Shah from SolarFlare:
> https://bugzilla.redhat.com/show_bug.cgi?id=896716
> (I was a bit optimistic in my initial review of the patches - there are
> actually a lot of issues that weren't handled by those patches.)
>> So I think this problem maybe could be solved by using the combination of existing
>> technology. and the following steps are we considering to implement:
>> -  before boot VM, we anticipate to specify two NICs for creating bonding device
>>     (one plugged and one virtual NIC) in XML. here we can specify the NIC's mac addresses
>>     in XML, which could facilitate qemu-guest-agent to find the network interfaces in guest.
> An interesting idea, but I think that is a 2nd level enhancement, not
> necessary initially (and maybe not ever, due to the high possibility of
> it being extremely difficult to get right in 100% of the cases).
>> -  when qemu-guest-agent startup in guest it would send a notification to libvirt,
>>     then libvirt will call the previous registered initialize callbacks. so through
>>     the callback functions, we can create the bonding device according to the XML
>>     configuration. and here we use netcf tool which can facilitate to create bonding device
>>     easily.
> This isn't quite making sense - the bond will be on the guest, which may
> not have netcf installed. Anyway, I think it should be up to the guest's
> own system network config to have the bond already setup. If you try to
> impose it from outside that infrastructure, you run too much risk of
> running afoul of something on the guest (e.g. NetworkManager)
>> -  during migration, unplug the passthroughed NIC. then do native migration.
> Correct. This is the most important part. But not just unplugging it,
> you also need to wait until the unplug operation completes (it is
> asynchronous). (After this point, the emulated NIC that is part of the
> bond would get all of the traffic).
>> -  on destination side, check whether need to hotplug new NIC according to specified XML.
>>     usually, we use migrate "--xml" command option to specify the destination host NIC mac
>>     address to hotplug a new NIC, because source side passthrough NIC mac address is different,
>>     then hotplug the deivce according to the destination XML configuration.
> Why does the MAC address need to be different? Are you suggesting doing
> this with passed-through non-SRIOV NICs? An SRIOV virtual function gets
> its MAC address from the libvirt config, so it's very simple to use the
> same MAC address across the migration. Any network card that would be
> able to do this on any sort of useful scale will be SRIOV-capable (or
> should be replaced with one that is - some of them are not that expensive).
Hi Laine,

I think SRIOV virtual NIC to support migration is good idea,
but I also think some passthrough NIC without SRIOV-capable. for
these NIC devices we only able to use <hostdev> to specify the passthrough
function, so for these NIC I think we should support too.


>> TODO:
>>    1.  when hot add a new NIC in destination side after migration finished, the NIC device
>>        need to re-enslave on bonding device in guest. otherwise, it is offline. maybe
>>        we should consider bonding driver to support add interfaces dynamically.
> I never looked at the details of how SolarFlare's code handled the guest
> side (they have/had their own patchset they maintained for some older
> version of libvirt which integrated with some sort of enhanced bonding
> driver on the guests). I assumed the bond driver could handle this
> already, but have to say I never investigated.
>> This is an example on how this might work, so I want to hear some voices about this scenario.
>> Thanks,
>> Chen
>> Chen Fan (7):
>>    qemu-agent: add agent init callback when detecting guest setup
>>    qemu: add guest init event callback to do the initialize work for
>>      guest
>>    hostdev: add a 'bond' type element in <hostdev> element
> Putting this into <hostdev> is the wrong approach, for two reasons: 1)
> it doesn't account for the device to be used being in a different
> address on the source and destination hosts, 2) the <interface> element
> already has much of the config you need, and an interface type
> supporting hostdev passthrough.
> It has been possible to do passthrough of an SRIOV VF via <interface
> type='hostdev'> for a long time now and, even better, via an <interface
> type='network'> where the network pointed to contains a pool of VFs - As
> long as the source and destination hosts both have networks with the
> same name, libvirt will be able to find a currently available device on
> the destination as it migrates from one host to another instead of
> relying on both hosts having the exact same device at the exact same
> address on the host and destination (and also magically unused by any
> other guest). This page explains the use of a "hostdev network" which
> has a pool of devices:
> http://wiki.libvirt.org/page/Networking#Assignment_from_a_pool_of_SRIOV_VFs_in_a_libvirt_.3Cnetwork.3E_definition
> This was designed specifically with the idea in mind that one day it
> would be possible to migrate a domain with a hostdev device (as long as
> the guest could handle the hostdev device being temporarily unplugged
> during the migration).
>>    qemu-agent: add qemuAgentCreateBond interface
>>    hostdev: add parse ip and route for bond configure
> Again, I think that this level of detail about the guest network config
> belongs on the guest, not in libvirt.
>>    migrate: hot remove hostdev at perform phase for bond device
> ^^ this is the useful part but I don't think the right method is to make
> this action dependent on the device being a "bond".
> I think that in this respect Shradha's patches had a better idea - any
> hostdev (or, by implication <interface type='hostdev'> or, much more
> usefully <interface type='network'> pointing to a pool of VFs - could
> have an attribute "ephemeral". If ephemeral was "yes", then the device
> would always be unplugged prior to migration and re-plugged when
> migration was completed (the same thing should be done when
> saving/restoring a domain which also can't currently be done with a
> domain that has a passthrough device).
> For that matter, this could be a general-purpose thing (although
> probably most useful for hostdevs) - just make it possible for *any*
> hotpluggable device to be "ephemeral"; the meaning of this would be that
> every device marked as ephemeral should be unplugged prior to migration
> or save (and libvirt should wait for qemu to notify that the unplug is
> completed), and re-plugged right after the guest is restarted.
> (possibly it should be implemented as an <ephemeral> *element* rather
> than attribute, so that options could be specified).
> After that is implemented and works properly, then it might be the time
> to think about auto-creating the bond (although again, my opinion is
> that this is getting a bit too intrusive into the guest (and making it
> more likely to fail - I know from long experience with netcf that it is
> all too easy for some other service on the system (ahem) to mess up all
> your hard work); I think it would be better to just let the guest deal
> with setting up a bond in its system network config, and if the bond
> driver can't handle having a device in the bond unplugging and plugging,
> then the bond driver should be enhanced).
>>    migrate: add hostdev migrate status to support hostdev migration
>>   docs/schemas/basictypes.rng   |   6 ++
>>   docs/schemas/domaincommon.rng |  37 ++++++++
>>   src/conf/domain_conf.c        | 195 ++++++++++++++++++++++++++++++++++++++---
>>   src/conf/domain_conf.h        |  40 +++++++--
>>   src/conf/networkcommon_conf.c |  17 ----
>>   src/conf/networkcommon_conf.h |  17 ++++
>>   src/libvirt_private.syms      |   1 +
>>   src/qemu/qemu_agent.c         | 196 +++++++++++++++++++++++++++++++++++++++++-
>>   src/qemu/qemu_agent.h         |  12 +++
>>   src/qemu/qemu_command.c       |   3 +
>>   src/qemu/qemu_domain.c        |  70 +++++++++++++++
>>   src/qemu/qemu_domain.h        |  14 +++
>>   src/qemu/qemu_driver.c        |  38 ++++++++
>>   src/qemu/qemu_hotplug.c       |   8 +-
>>   src/qemu/qemu_migration.c     |  91 ++++++++++++++++++++
>>   src/qemu/qemu_migration.h     |   4 +
>>   src/qemu/qemu_process.c       |  32 +++++++
>>   src/util/virhostdev.c         |   3 +
>>   18 files changed, 745 insertions(+), 39 deletions(-)
> .

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