Subject: Re: Is fallback vhost_net to qemu for live migrate available？

From: Qin Chuanyu <[email-masked]>
Date: Sat, 31 Aug 2013 12:45:11 +0800

On 2013/8/30 0:08, Anthony Liguori wrote:
> Hi Qin,

>> By change the memory copy and notify mechanism ，currently virtio-net with
>> vhost_net could run on Xen with good performance。
>
> I think the key in doing this would be to implement a property
> ioeventfd and irqfd interface in the driver domain kernel.  Just
> hacking vhost_net with Xen specific knowledge would be pretty nasty
> IMHO.
>
Yes, I add a kernel module which persist virtio-net pio_addr and msix 
address as what kvm module did. Guest wake up vhost thread by adding a 
hook func in evtchn_interrupt.

> Did you modify the front end driver to do grant table mapping or is
> this all being done by mapping the domain's memory?
>
There is nothing changed in front end driver. Currently I use 
alloc_vm_area to get address space， and map the domain's memory as what 
what qemu did.

> KVM and Xen represent memory in a very different way.  KVM can only
> track when guest mode code dirties memory.  It relies on QEMU to track
> when guest memory is dirtied by QEMU.  Since vhost is running outside
> of QEMU, vhost also needs to tell QEMU when it has dirtied memory.
>
> I don't think this is a problem with Xen though.  I believe (although
> could be wrong) that Xen is able to track when either the domain or
> dom0 dirties memory.
>
> So I think you can simply ignore the dirty logging with vhost and it
> should Just Work.
>
Thanks for your advice, I have tried it, without ping, it could migrate 
successfully, but if there has skb been received, domU would crash. I 
guess that because though Xen track domU memory, but it could only track 
memory that changed in DomU. memory changed by Dom0 is out of control.

>
> No, we don't have a mechanism to fallback  to QEMU for the datapath.
> It would be possible but I think it's a bad idea to mix and match the
> two.
>
Next I would try to fallback datapath to qemu for three reason:
1: memory translate mechanism has been changed for vhost_net on Xen，so 
there would be some necessary changed needed for vhost_log in kernel.

2: I also maped IOREQ_PFN page(which is used for communication between 
qemu and Xen) in kernel notify module, so it also needed been marked as 
dirty when tx/rx exist in migrate period.

3: Most important of all， Michael S. Tsirkin said that he hadn't 
considered about vhost_net migrate on Xen，so there would be some 
changed needed in vhost_log for qemu.

fallback to qemu seems to much easier, isn't it.

Regards
Qin chuanyu