Re: [Qemu-devel] live migration vs device assignment (motivation)

[Michael S. Tsirkin]

On Thu, Dec 10, 2015 at 12:26:25AM +0800, Lan, Tianyu wrote:
> On 12/8/2015 12:50 AM, Michael S. Tsirkin wrote:
> >I thought about what this is doing at the high level, and I do have some
> >value in what you are trying to do, but I also think we need to clarify
> >the motivation a bit more.  What you are saying is not really what the
> >patches are doing.
> >
> >And with that clearer understanding of the motivation in mind (assuming
> >it actually captures a real need), I would also like to suggest some
> >changes.
> 
> Motivation:
> Most current solutions for migration with passthough device are based on
> the PCI hotplug but it has side affect and can't work for all device.
> 
> For NIC device:
> PCI hotplug solution can work around Network device migration
> via switching VF and PF.

This is just more confusion. hotplug is just a way to add and remove
devices. switching VF and PF is up to guest and hypervisor.

> But switching network interface will introduce service down time.
> 
> I tested the service down time via putting VF and PV interface
> into a bonded interface and ping the bonded interface during plug
> and unplug VF.
> 1) About 100ms when add VF
> 2) About 30ms when del VF

OK and what's the source of the downtime?
I'm guessing that's just arp being repopulated.  So simply save and
re-populate it.

There would be a much cleaner solution.

Or maybe there's a timer there that just delays hotplug
for no reason. Fix it, everyone will benefit.

> It also requires guest to do switch configuration.

That's just wrong. if you want a switch, you need to
configure a switch.

> These are hard to
> manage and deploy from our customers.

So kernel want to remain flexible, and the stack is
configurable. Downside: customers need to deploy userspace
to configure it. Your solution: a hard-coded configuration
within kernel and hypervisor.  Sorry, this makes no sense.
If kernel is easier for you to deploy than userspace,
you need to rethink your deployment strategy.

> To maintain PV performance during
> migration, host side also needs to assign a VF to PV device. This
> affects scalability.

No idea what this means.

> These factors block SRIOV NIC passthough usage in the cloud service and
> OPNFV which require network high performance and stability a lot.

Everyone needs performance and scalability.

> 
> For other kind of devices, it's hard to work.
> We are also adding migration support for QAT(QuickAssist Technology) device.
> 
> QAT device user case introduction.
> Server, networking, big data, and storage applications use QuickAssist
> Technology to offload servers from handling compute-intensive operations,
> such as:
> 1) Symmetric cryptography functions including cipher operations and
> authentication operations
> 2) Public key functions including RSA, Diffie-Hellman, and elliptic curve
> cryptography
> 3) Compression and decompression functions including DEFLATE and LZS
> 
> PCI hotplug will not work for such devices during migration and these
> operations will fail when unplug device.
> 
> So we are trying implementing a new solution which really migrates
> device state to target machine and won't affect user during migration
> with low service down time.

Let's assume for the sake of the argument that there's a lot going on
and removing the device is just too slow (though you should figure out
what's going on before giving up and just building something new from
scratch).

I still don't think you should be migrating state.  That's just too
fragile, and it also means you depend on driver to be nice and shut down
device on source, so you can not migrate at will.  Instead, reset device
on destination and re-initialize it.

-- 
MST