Re: [virtio-comment] [RFC PATCH 0/5] virtio: introduce SUSPEND bit and vq state

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Aug 15, 2023 at 03:28:59AM +0800, Zhu Lingshan wrote:
> This seires introduces
> 1)a new SUSPEND bit in the device status
> Which is used to suspend the device, so that the device states
> and virtqueue states are stablized.
> 
> 2)virtqueue state and accessors, to get and set last_avail_idx
> and last_used_idx of virtqueues.
> 
> The main usecase of these new facilities is Live Migration.
> 
> Furture work: dirty page tracking and in-flight descriptors.
> 
> This is RFC, this series carries on Jason and Eugenio's pervious work.
> 
> Any comments are welcome.

In order to support stateful devices like virtiofs, virtio-gpu, or
virtio-crypto it would be necessary to add device state save/load
functionality too.

Even virtio-net needs device state save/load functionality if the VMM is
not intercepting the stateful controlq.

In-flight descriptors can be included in the device state.

In fact, the only reason why I think VIRTIO_F_QUEUE_STATE makes sense is
for easy integration into existing vhost software stacks. Otherwise we
should just define a device state save/load interface along with
standard device state serialization for devices where migration
interoperability is possible. VIRTIO_F_QUEUE_STATE is a small subset of
device state save/load.

I think the convenience of integrating into existing vhost software
stacks is worth the duplication, but I wanted to mention that this
interface will not be enough to live migrate all device types and we'll
need something more in the future.

Stefan

> 
> Zhu Lingshan (5):
>   virtio: introduce SUSPEND bit in device status
>   virtio: introduce vq state as basic facility
>   virtio: The actions by the device upon SUSPEND
>   virtqueue: constraints for virtqueue state
>   virtio-pci: implement VIRTIO_F_QUEUE_STATE
> 
>  content.tex       | 120 ++++++++++++++++++++++++++++++++++++++++++++++
>  transport-pci.tex |  15 ++++++
>  2 files changed, 135 insertions(+)
> 
> -- 
> 2.35.3
> 
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