Re: [Qemu-devel] [RFC PATCH V2 00/10] Qemu: Add live migration support for SRIOV NIC

[Michael S. Tsirkin]

On Tue, Nov 24, 2015 at 09:35:17PM +0800, Lan Tianyu wrote:
> This patchset is to propose a solution of adding live migration
> support for SRIOV NIC.
> 
> During migration, Qemu needs to let VF driver in the VM to know
> migration start and end. Qemu adds faked PCI migration capability
> to help to sync status between two sides during migration.
> 
> Qemu triggers VF's mailbox irq via sending MSIX msg when migration
> status is changed. VF driver tells Qemu its mailbox vector index
> via the new PCI capability. In some cases(NIC is suspended or closed),
> VF mailbox irq is freed and VF driver can disable irq injecting via
> new capability.   
> 
> VF driver will put down nic before migration and put up again on
> the target machine.

It is still not very clear what it is you are trying to achieve, and
whether your patchset achieves it.  You merely say "adding live
migration" but it seems pretty clear this isn't about being able to
migrate a guest transparently, since you are adding a host/guest
handshake.

This isn't about functionality either: I think that on KVM, it isn't
hard to live migrate if you can do a host/guest handshake, even today,
with no kernel changes:
1. before migration, expose a pv nic to guest (can be done directly on
  boot)
2. use e.g. a serial connection to move IP from an assigned device to pv nic
3. maybe move the mac as well
4. eject the assigned device
5. detect eject on host (QEMU generates a DEVICE_DELETED event when this
   happens) and start migration

Is this patchset a performance optimization then?
If yes it needs to be accompanied with some performance numbers.

-- 
MST