Re: [virtio-comment] [PATCH V2 0/2] Introduce VIRTIO_F_QUEUE_STATE

[Jason Wang <jasowang@redhat.com>]

å 2021/3/23 äå6:40, Stefan Hajnoczi åé:
> On Mon, Mar 22, 2021 at 11:47:15AM +0800, Jason Wang wrote:
> > This is a new version to support VIRTIO_F_QUEUE_STATE. The feautre
> > extends the basic facility to allow the driver to set and get device
> > internal virtqueue state. This main motivation is to support live
> > migration of virtio devices.
> Can you describe the use cases that this interface covers as well as the
> steps involved in migrating a device?


Yes. I can describe the steps for live migrating virtio-net device. For 
other devices, we probably need other state.


>   Traditionally live migration was
> transparent to the VIRTIO driver because it was performed by the
> hypervisor.


Right, but it could be possible that we may want live migrate between 
hardware virtio-pci devices. So it's up to the hypversior to save and 
restore states silently without the notice of guest driver as what we 
did for vhost.


> I know you're aware but I think it's worth mentioning that this only
> supports stateless devices.


Yes, that's why it's a queue state not a device state.


>   Even the simple virtio-blk device has state
> in QEMU's implementation. If an I/O request fails it can be held by the
> device and resumed after live migration instead of failing the request
> immediately. The list of held requests needs to be migrated with the
> device and is not part of the virtqueue state.


Yes, I think we need to extend virtio spec to support save and restore 
device state. But anyway the virtqueue state is the infrastructure which 
should be introdouced first.


> I'm concerned that using device reset will not work once this interface
> is extended to support device-specific state (e.g. the virtio-blk failed
> request list). There could be situations where reset really needs to
> reset (e.g. freeing I/O resources) and the device therefore cannot hold
> on to state across reset.


Good point. So here're some ways:


1) reuse device reset that is done in this patch
2) intorduce a new device status like what has been done in [1]
3) using queue_enable (as what has been done in the virtio-mmio, pci 
forbids to stop a queue currently, we may need to extend that)
4) use device specific way to stop the datapath

Reusing device reset looks like a shortcut that might not be easy for 
stateful device as you said. 2) looks more general. 3) have the issues 
that it doesn't forbid the config changed. And 4) is also proposed by 
you and Michael.

My understanding is that there should be no fundamental differences 
between 2) and 4). So I tend to respin [1], do you have any other ideas?

Thanks

[1] 
https://lists.oasis-open.org/archives/virtio-comment/202012/msg00027.html


> Stefan