Re: [Qemu-devel] Logging dirty pages from vhost-net in-kernel with vIOMMU

[Jason Wang <jasowang@redhat.com>]

On 2018/12/5 下午10:47, Jintack Lim wrote:
> On Tue, Dec 4, 2018 at 8:30 PM Jason Wang <jasowang@redhat.com> wrote:
>>
>> On 2018/12/5 上午2:37, Jintack Lim wrote:
>>> Hi,
>>>
>>> I'm wondering how the current implementation works when logging dirty
>>> pages during migration from vhost-net (in kernel) when used vIOMMU.
>>>
>>> I understand how vhost-net logs GPAs when not using vIOMMU. But when
>>> we use vhost with vIOMMU, then shouldn't vhost-net need to log the
>>> translated address (GPA) instead of the address written in the
>>> descriptor (IOVA) ? The current implementation looks like vhost-net
>>> just logs IOVA without translation in vhost_get_vq_desc() in
>>> drivers/vhost/net.c. It seems like QEMU doesn't do any further
>>> translation of the dirty log when syncing.
>>>
>>> I might be missing something. Could somebody shed some light on this?
>>
>> Good catch. It looks like a bug to me. Want to post a patch for this?
> Thanks for the confirmation.
>
> What would be a good setup to catch this kind of migration bug? I
> tried to observe it in the VM expecting to see network applications
> not getting data correctly on the destination, but it was not
> successful (i.e. the VM on the destination just worked fine.) I didn't
> even see anything going wrong when I disabled the vhost logging
> completely without using vIOMMU.
>
> What I did is I ran multiple network benchmarks (e.g. netperf tcp
> stream and my own one to check correctness of received data) in a VM
> without vhost dirty page logging, and the benchmarks just ran fine in
> the destination. I checked the used ring at the time the VM is stopped
> in the source for migration, and it had multiple descriptors that is
> (probably) not processed in the VM yet. Do you have any insight how it
> could just work and what would be a good setup to catch this?


According to past experience, it could be reproduced by doing scp from 
host to guest during migration.


>
> About sending a patch, as Michael suggested, I think it's better for
> you to handle this case - this is not my area of expertise, yet :-)


No problem, I will fix this.

Thanks for spotting this issue.


>> Thanks
>>
>>
>>> Thanks,
>>> Jintack
>>>
>>>